Nistchempy

- Adds c.a. 10 000 of missing InChI strings to the pre-saved data on compounds (`nistchempy.compound_list.get_all_data`).
- Fixes bug in update resulted in c.a. 10 000 of missing InChI strings.
- Fixes chemical formula parser.
- Adds reference to repo containing data extracted from NIST Chemistry WebBook ([NistChemData](https://github.com/IvanChernyshov/NistChemData)).

Fixes several bugs:

* error why saving spectra via the `nistchepy.compound.NistCompound.save_spectra` method;

* removes unintended spaces in chemical formula.

The first stable release.