Tempo implementation update

• Tempo and empty tokens are implemented for compatible encodings
• CP Word implement "Ignore" tokens by default as in the original paper
• Removed Chord option from Structured encoding (compatible but breaks the "structure")
• current_midi_metadata in MIDITokenizer base class allows every child method to access directly to these values (tempo changes, time division) without passing them in parameters

MuMIDI and Octuple encodings

• MuMIDI encoding implemented
• Octuple encoding implemented
• load_params method in MIDITokenizer allow to create a tokenizer from parameters saved in a file
• small code improvements and fixes, some methods / variables renamed with better names

Initial release

• MIDITokenizer base class completed
• First encoding strategies: Remi, CP word, Structured, MIDI-Like
• First additional token types: chords, empty bar
• Docstring completed