The build context is now passed as a mutable reference due to the need
to mutate data when adding a model.
Contains small unrelated cleanups and documentation improvements too.
The external_input example has been as well adapted and (at least
temporarily) simplifiedi/modified to remove the dependencies on
`atomic_wait` and `mio`.
This makes it possible to concurrently control and monitor the
simulation when using gRPC.
Accordingly, the gRPC server now runs on 2 threads so it can serve
control and monitoring requests concurrently.
This is a pretty large patch that impacts the API.
Until now, it was not possible to cancel events that were scheduled for
the current simulation time slice, making it necessary for the user to
use complex workarounds (see former version of the espresso machine
example).
The new implementation makes this possible but the generation of a key
associated to an event has now a non-negligible cost (basicaly it
creates three references to an Arc). For this reason, the API now
defaults to NOT creating a key, and new methods were added for
situations when the event may need to be cancelled and a key is
necessary.
See the much simplified implementation of the espresso machine example
for a motivating case.
