release 0.0.1 of fsfw added as a core

fsfw/contrib/sgp4/LICENSE

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+There is no license associated with the code and you may use it for any purpose —personal or commercial— as you
+wish. We ask only that you include citations in your documentation and source code to show the source of the code
+and provide links to https://www.celestrak.com/publications/AIAA/2006-6753/, to facilitate communications
+regarding any questions on the theory or source code.

fsfw/contrib/sgp4/sgp4unit.h

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+#ifndef _sgp4unit_
+#define _sgp4unit_
+/*     ----------------------------------------------------------------
+*
+*                                 sgp4unit.h
+*
+*    this file contains the sgp4 procedures for analytical propagation
+*    of a satellite. the code was originally released in the 1980 and 1986
+*    spacetrack papers. a detailed discussion of the theory and history
+*    may be found in the 2006 aiaa paper by vallado, crawford, hujsak,
+*    and kelso.
+*
+*                            companion code for
+*               fundamentals of astrodynamics and applications
+*                                    2007
+*                              by david vallado
+*
+*       (w) 719-573-2600, email dvallado@agi.com
+*
+*    current :
+*              20 apr 07  david vallado
+*                           misc fixes for constants
+*    changes :
+*              11 aug 06  david vallado
+*                           chg lyddane choice back to strn3, constants, misc doc
+*              15 dec 05  david vallado
+*                           misc fixes
+*              26 jul 05  david vallado
+*                           fixes for paper
+*                           note that each fix is preceded by a
+*                           comment with "sgp4fix" and an explanation of
+*                           what was changed
+*              10 aug 04  david vallado
+*                           2nd printing baseline working
+*              14 may 01  david vallado
+*                           2nd edition baseline
+*                     80  norad
+*                           original baseline
+*       ----------------------------------------------------------------      */
+
+#include <math.h>
+#include <stdio.h>
+
+// -------------------------- structure declarations ----------------------------
+typedef enum
+{
+  wgs72old,
+  wgs72,
+  wgs84
+} gravconsttype;
+
+typedef struct elsetrec
+{
+  long int  satnum;
+  int       epochyr, epochtynumrev;
+  int       error;
+  char      init, method;
+
+  /* Near Earth */
+  int    isimp;
+  double aycof  , con41  , cc1    , cc4      , cc5    , d2      , d3   , d4    ,
+         delmo  , eta    , argpdot, omgcof   , sinmao , t       , t2cof, t3cof ,
+         t4cof  , t5cof  , x1mth2 , x7thm1   , mdot   , nodedot, xlcof , xmcof ,
+         nodecf;
+
+  /* Deep Space */
+  int    irez;
+  double d2201  , d2211  , d3210  , d3222    , d4410  , d4422   , d5220 , d5232 ,
+         d5421  , d5433  , dedt   , del1     , del2   , del3    , didt  , dmdt  ,
+         dnodt  , domdt  , e3     , ee2      , peo    , pgho    , pho   , pinco ,
+         plo    , se2    , se3    , sgh2     , sgh3   , sgh4    , sh2   , sh3   ,
+         si2    , si3    , sl2    , sl3      , sl4    , gsto    , xfact , xgh2  ,
+         xgh3   , xgh4   , xh2    , xh3      , xi2    , xi3     , xl2   , xl3   ,
+         xl4    , xlamo  , zmol   , zmos     , atime  , xli     , xni;
+
+  double a      , altp   , alta   , epochdays, jdsatepoch       , nddot , ndot  ,
+         bstar  , rcse   , inclo  , nodeo    , ecco             , argpo , mo    ,
+         no;
+} elsetrec;
+
+// --------------------------- function declarations ----------------------------
+int sgp4init
+       (
+         gravconsttype whichconst,      const int satn,     const double epoch,
+         const double xbstar, const double xecco, const double xargpo,
+         const double xinclo, const double xmo,   const double xno,
+         const double xnodeo,
+         elsetrec& satrec
+       );
+
+int sgp4
+       (
+         gravconsttype whichconst,
+         elsetrec& satrec, double tsince,
+         double r[], double v[]
+       );
+
+double  gstime
+       (
+         double
+       );
+
+void getgravconst
+      (
+       gravconsttype,
+       double&,
+       double&,
+       double&,
+       double&,
+       double&,
+       double&,
+       double&,
+       double&
+      );
+
+#endif
+