Create object describing electron configuration and energy level of atom

[lemmatum]

This object should do things like return the number of electrons in a shell, return a string in spectroscopic notation for the configuration. Two configuration objects can then be used to describe a transition that emits/absorbs a photon and lookup parameters in a database (e.g., Einstein's A and B coefficients, statistical weights, energy of emitted/absorbed photon). An entire list of these objects can be used to select which transitions and energy levels to consider when simulating spectra (e.g. when simulating high temperature plasmas that end up predominantly in the He-like state, we can ignore all but the ionization from ground transitions for each charge state before He-like as the contributions will be negligible, but would add considerably to computation time).

[lemmatum]

Some overlap with fiasco, e.g., IonBase class in base.py, though I think this will be a more detailed description which includes energy levels and not just charge states.

[lemmatum]

I've currently got a class for describing subshells and the number of electrons contained in a subshell. I've also outlined classes for electron configurations and term symbols. These two combined will give a unique description of the energy level (probably another class combining the configuration and term). And then any 2 energy levels will form a transition.