Define Configuration Schema for Ion Species

[psktam]

Currently, ion species are configured directly as Julia code in the src/species/<xxx>.jl files. Prefer encoding the configurations separately as read-only (ie: non-executable) files in their own dedicated location in the repository.

Suggest re-implementing these configurations in a config-only format. YAML is highly attractive as it's a relatively flexible format and is also easy to use, and there seems to be a Julia package available to parse YAML files: https://juliapackages.com/p/yaml

[psktam]

Discussions had while developing for this issue led to additional work being done to remove IonInstance and the species-specific constructors in order to get rid of the constructor boilerplate and to clean up the class structure.