Feature: better load / save support

[mzschwartz5]

Prevent need for revoxelizing everything by saving node data to file / reloading on file open. This was already partially accounted for when doing the multi-mesh refactor; everything is already structured in nodes / data classes. Just needs to be taken across the finish line.

[mzschwartz5]

Data classes will need to have both Binary and ASCII methods to support both .ma and .mb filetypes

[mzschwartz5]

GPU buffers that need to persist (like paint weights) will need to be saved to attributes (should be in place already where needed) and then reuploaded to the GPU somehow from those attributes on file load. (I say "somehow" - the hard part is wiring things back up: i.e. how the data gets from the MPxData instance to the shader.