add end() method to stop all communication with the Transport channel

[lathoub]

The library provides a begin() method to open the communication with the Transport layer, but no opposite method is provided (end()) to close the communication with the Transport layer.

Use-case: an interruption in the Transport protocol, causing a breakdown of the Transport layer can't be fixed by restarting the MIDI object - due to the missing end() method.

A restart of the Transport layer could be forced by calling MIDI.end(); MIDI.begin();

Impact: all underlying Transport mechanisms have to implement the end() method, so the impact on backward compatibility (on the Transport layers) and dependancy on a specific version of this library is significant and needs to be planned carefully.

[lathoub]

Added PR #218 for the end() method for the Serial Transport layer

[franky47]

Merged !

[lathoub]

Thanks @franky47

All Transport layers now have the end() method, that will need to be called by the MIDI protocol in MIDI.h:

    void end()
    {
        mTransport.end();
    };

The code itself is simple to add, but creates a backward compatibility problem. The Transport layers now become dependent on a specific version of this MIDI library. AFAIK the depends clause in the library.properties does not include version (does it?).

Possible solution to the above version issue is to sequence the release in order: this library needs to release first, once that is done, then the Transport layers can release, depending on the just released MIDI library. (Nothing is on fire, so no need to rush the release)

Once we agree on an approach, I can issue the PR for the MIDI end() method.