Mutwo.ext-midi

Added
- backends module to convert midi files to mutwo event (experimental state):
- `MidiFileToEvent`
- `MidiVelocityToWesternVolume`
- `MidiVelocityToMutwoVolume`
- `MidiPitchToMutwoMidiPitch`
- `MidiPitchToDirectPitch`
- `MidiPitchToMutwoPitch`
- `PitchBendingNumberToDirectPitchInterval`
- `PitchBendingNumberToPitchInterval`

Changed
- some variables in `midi_converters.constants` to `midi_converters.configurations`

Fixed
- slow calculation for glissandi / pitch envelopes

Changed
- package structure to namespace package to apply refactor of mutwo main package

Changed
- allow endless nested sequential events (if no simultaneous events occur in between)

Added
- support for pitch envelopes (allow glissandi now!)

Changed
- applied movement from music related parameter and converter modules (which have been moved from [mutwo core](https://github.com/mutwo-org/mutwo) in version 0.49.0 to [mutwo.ext-music](https://github.com/mutwo-org/mutwo.ext-music))