Constrained-matrix-factorization

Enable partial initialization and constraint specification
- Address scope and convenience imports. The nmf module is accesible at the top level, as well as the NMF class.
:code:`cmf.nmf...` and :code:`cmf.NMF(...)`.
- More flexibility at torch level with constraints. Allows for partial initialization of components and weights, and
partial specification of constraints.

- Refactor of companions and wrappers: wrappers wrap functionality; companions have :code:`ask, tell, report` methods.
- CUDA and GPU Functionality added to base classes, utility functions, and total scattering
- Required kwargs on functions, instead of allowing positional.
- Numerical instability patch for when positive comps (mu denominator in multiplicative update) are 0.
- Added much more documentation.