Created static metrics model_size, connection_sparsity, and parameter_count

[EssentiallyNoah]

55 and #54

Created net function for NeuroBenchModel and updated snntorch_models.py and torch_models.py. Added unit tests. Also created static metric parameter_count which returns a count of the number of parameters separate from the amount of space they take in memory.

[jasonlyik]

Closes #55 Closes #54