REFACTOR: GaussExc.py dealing with Tukey axial weighting

[mlp6]

1. Docstrings for input arguments (e.g., sigma) are lacking.
2. Need to explicitly test / warn the user that sigma should only be two elements when the Tukey window is active (i.e., a z value for sigma has no meaning and should not be specified in that case).
3. Add logging about choosing the gauss vs. tukey load distributions (maybe have the user explicitly choose rather than implicitly figuring it out with the specification of a Tukey-based parameter.