New tutorial and automated tests

This release provides improved documentation all around, with an all new Jupyter notebook tutorial that explains some of the math in relation to the code.

Additionally, there is now an automated test suite that verifies that all core functions are working properly, finishing with a numerical comparison against a reference data set.

Improved parsing and documentation

Smarter input and chi2 parsing, improved general documentation and README.

First production release

SHGYield is a python program for calculating the second-harmonic generation (SHG) yield for semiconductor surfaces. This is useful for conducting theoretical second-harmonic spectroscopy of any crystalline semiconductor or nanostructure.