Arc-alkali-rydberg-calculator

Introduces `arc.advanced` collection of modules that extend basic ARC functionality. Modules are contributed by the research community (and we are looking forward to more contributions!).

The first contribution is by Alessandro Greco, Dipartimento di Fisica “E. Fermi”, Università di Pisa, Italy. He created function `getPopulationLifetime` for calculating population lifetimes in Rydberg manifold and that can be loaded from `arc.advanced.population_lifetime` . See [documentation](https://arc-alkali-rydberg-calculator.readthedocs.io/en/latest/advanced.html) and [readme](/arc/advanced/README.md) for more details

Updates dependencies with Numpy (>=1.16.3) and Scipy so that new versions of libraries can be used (26).

Minor release fixes selection of basis in pair-state basis for calculations of very low states (like first excited state).
See 4e1ddbbe3075e3315469153a29b871667cb01d31 and d08ea621db8b9922f099223a195998dce86e4c92 .

Updated literature values for Caesium with values in Phys. Rev. A 94, 012505 (2016).

**For updating existing ARC version:**

* if you want to **keep your calculated dme** you have to manually copy `/arc/data/caesium_literature_dme.csv` into `~/.arc-data` on your computer;
* if you do **not want to keep your calculated dme** you can delete `~/.arc-data` folder, and on first loading of new folder will be created and populated.

Minor fix: getStateLifetime to include decays to levels that are for smaller
principal quantum number (n) than ground level, but are above in energy
due to angular part (see 5a73af2c27d0bee90ad8007a66d432b8a36ef28b)

This release does not introduce any changes to the code base.

Just forces wheels (distributed in installation with pip install...) to be build with old Numpy to resolve installation issues reported in 25 .