Look up in cherab.core.atomic.elements instead of using _popular_species and _species_symbol_map dicts

[vsnever]

I suggest that instead of using _popular_species dict when reading the data from raw or balance files, we should search for the matches in cherab.core.atomic.elements. First, get all isotopes with a given nuclear charge, second, from them find the one with the closest atomic mass to the given one.

Similarly, _species_symbol_map dict must be replaced with the search for the matching element symbol in cherab.core.atomic.elements. Extracting the nuclear charge from the balance file also must be improved the same way.

[CnlPepper]

I built these search functions to simplify element handling:

https://github.com/cherab/core/blob/d58c8c26470209be9643a8c23b9dc9385bbe30e9/cherab/core/atomic/elements.pyx#L136

You can lookup the element by name, atomic number etc....

[vsnever]

Yes @CnlPepper, it's exactly what I was looking for, thank you. I'll update the code and make the PR then.

@jacklovell, could you please provide me with any balance.nc file for testing? All examples in SOLPS-ITER are in md5.

[vsnever]

Current representation of species in SOLPSSimulation class as a list of strings (symbol + charge) in unsafe. Let's say we have He3 in the plasma, for the neutral He3, re.match(_SPECIES_REGEX, sp).groups() will return ('He', 30). It's better to keep the species as the list of (symbol, charge) tuples or even (Element/Isotope, charge) tuples. In the latter case, we can avoid try-except for Element/Isotope when initialising Species.

[vsnever]

Fixed in #35.