Add: MIDIfile save function and new requirement

[gregory-mc]

I added a function to save midifile that includes parts of play_midi_from_data( ) and write_to_midifile( ) I find this interesting when someone wants to sonify data with loops (76 items for example).

I also added a new requirement because numpy version 1.20 requires python 3.7. I tried install sonify in a conda env with python 3.7 and I couldn't.