[BUG] Invalid escape sequence \g in theoretical variogram models #510

Package version
1.2.0

Describe the bug
pyinterpolate/semivariogram/theoretical/variogram_models/models.py:262: SyntaxWarning: invalid escape sequence '\g'

To Reproduce
Steps to reproduce the behavior:

1. Run python script dependent on pyinterpolate on heroku server.

Input Data Schema
n/a

Expected behavior
No warnings

Screenshots
n/a

Desktop (please complete the following information):

• OS: Ubuntu

Additional context
Probably docstrings must be slightly changed (because there are latex equations starting with \g - \gamma - for example:

$\gamma = c0 + c * (1 - \exp(-\frac{a^{2}}{h^{2}}))$, $a > 0$