chaining for all commands, callbacks and setters

[lathoub]

modified all commands, callbacks and setters with no return value (void) to return a reference to this.

example:

template<class Transport, class Settings, class Platform>
void MidiInterface<Transport, Settings, Platform>::begin(Channel inChannel)
{
...
}

becomes

template<class Transport, class Settings, class Platform>
MidiInterface<Transport, Settings, Platform>& MidiInterface<Transport, Settings, Platform>::begin(Channel inChannel)
{
...
    return *this;
}

this modif has no impact on existing code, and code size

[coveralls]

Coverage decreased (-0.008%) to 99.624% when pulling 417beca on chaining into b4daa69 on master.

[lathoub]

Not sure how to fix the platformIO errors

[franky47]

It's under discussion here, I'll have a look, it's nothing related to this PR.

[franky47]

Apart from the internal method, LGTM.

[lathoub]

added example on how to use command chaining with lambda functions