Add docstrings and refactor code

[qpenko]

This is an attempt to:

• Add docstrings to every function. Existing code comments and online resources were used to obtain function and parameters description
• Refactor code to comply with PEP8 where possible
• Update code for some routines to make it consistent across the package
• Fix bugs, remove unused variables and duplicated functions

Areas that need additional attention were marked with "TODO" comment:

• Docstring parameters without description
• Unused variables
• Unknown function

[GeostatsGuy]

Appreciation to Anton Kupenko for this great assistance on the package. Looking forward to working together on this package. I will add missing parameter descriptions in the next week. I'm almost done with sequential Gaussian simulation. Michael