Renaming in SparseStochMat module and others

[j-i-l]

The convention is (scipy is actually a terrible example for this):

• snake_case for module names
• PascalCase for class names

so:

SparseStochMat.py -> sparse_stoch_mat.py class sparse_stoch_mat -> class SparseStochMat (I'd even go for just SSM)

There are other namings that we might want to change:

• sub-module SynthTempNetwork.py > synth_temp_network.py

And some we might want to keep (like Upper_case names for matrix-like objects).