Ndispers

New feature:
- Method, `deff_sfg` method was added to the BetaBBO classes. This computes the wavelength dependence of nonlinear coefficients based on Miller's rule.
- Point-group class, `Uniax_neg_3m`, was created, inherited from `Medium` class. BetaBBO classes are then inherited from `Uniax_neg_3m`. This class is only used internally to generalize `deff_sfg` method to all the different BetaBBO modules.

New crystals:
- SLN : 1% MgO-doped stoichiometric Lithium niobate (Li Nb O_3) crystal
- KBBF

Fixed:

- Dependancy issue: mpmath is now required as external of sympy

Printing sympy expressions

- Wavelength symbol was changed from wl to lambda so that it is printed as a Greek letter in Jupyter notebook/lab

- A new LBO module, LBO_KK1994, based on K. Kato's paper published in 1994.
- Bug fix: Group delay (GD) method in _baseclass.py
- Some minor updates in documentation

Bugs in n_e_expr for yz and zx planes of biaxial crystal modules (LBO, KTP) were corrected.

LBO modules were updates, tested and organized. Note that the module names were changed from the previous release v0.1.13. Also the beta-BBO module name were changed. The method name "pmBand_sfg" in v0.1.13 was changed to "pmFactor_sfg".

The first release to PyPI.