Support ion object creation with roman numeral notation

[wtbarnes]

Currently, ion objects can only be created using Arabic numerals, e.g.

import fiasco
fiasco.IonBase('Fe 5')
fiasco.IonBase('Fe +4')

Given that ions are commonly denoted using roman numerals, i.e. Fe V is Fe 5 or Fe +4, it would be nice to be able create ions as

fiasco.IonBase('Fe V')

[namurphy]

We recently added this capability for Particle class instances in PlasmaPy, so we can now do things like:

>>> from plasmapy.atomic import Particle
>>> O_VI = Particle("O VI")
>>> O_VI.ionic_symbol
'O 5+'

This was implemented using the roman package in PlasmaPy/PlasmaPy#290, in case you'd like to take a look.

[wtbarnes]

Oh that is neat. Thanks for the heads up! I actually already use the plasmapy.atomic module to handle all of the different ways to create an ion or element with a string representation (see the ion Base class) so presumably this new functionality would be easy to pull in.