Arc-alkali-rydberg-calculator

Resolves bug where initialising two different isotopes of the same element will overwrite their HFS parameters in runtime database - see more details in 93 (many thanks to dihm for reporting this)

Minor bug fix release: fixes formatting of Potassium data .csv file (5af105621a91f1f1cb5f4278d85820db34cf0ca7) that was preventing Potassium instance creation (see issue 92 )

Improved documentation.
Significant overhaul of documentation is done, in order to make it more accessible.

Resolves 89

This release fixes important bugs, and adds new functionality:

* Adds support for hyperfine structure calculations (many thanks to jonathanpritchard 87 ) - see [ARC_3_1_Additions.ipynb](https://github.com/nikolasibalic/ARC-Alkali-Rydberg-Calculator/blob/master/doc/ARC_3_1_Additions.ipynb) or [ARC_3_1_Additions.html](https://github.com/nikolasibalic/ARC-Alkali-Rydberg-Calculator/blob/master/doc/_static/ARC_3_1_Additions.html) for more details
* Fixes bug that caused low-ell states not to be included when calculating quadrupole-quadrupole interactions in the PairStateInteractions (many thanks to SylvainDLDK 81 )
* Fixes wrong energy units returned by StarkMap.getState (many thanks to Dobrzyniecki 85 )
* Improves precision of high \ell states by incorporating scalling of quantum defects for \ell >4 (many thanks to rbrown137 80)
* Adds support for formatting SI prefixes in LevelPlot (many thanks to petermao 71 )

This release fixes the bug with Calcium40 Quantum defects that was affect 1S_0, 1P_1 and 1F_3 states (see issue 78 )