v0.4.0-beta.4.0

final release of beta 4.0, please read the README in main to get caught up with current changes to MIDIAnimator

Some of the changes include:
New keyframing system
Instead of sampling the animation for each frame, keyframes are duplicated, adding their frame times together.

Note Off support:
Allows you to play sustaining chords

Velocity support:
Soft velocities will have decreased amplitude on the animation, higher velocities will have an increased amplitude

X & Y mappers:
Allows you to manipulate some of the animation characteristics directly. For instance, a weighted, heavy vibraphone bar will not respond the same way as a higher pitched, lighter bar.
This allows you to “map” the lower notes to respond slower, and the higher notes to respond quicker (by shortening the time delta)

internal changes, including allowing Blender properties to be on Instrument class instances, and hosting UI code inside of Instrument classes

way better error handling and sanitizing user input

some things are unfinished (like oscillations, ADSR, etc) but will work without it