Separate nuclear data from the concept of an Isotope

[MicahGale]

Right now the Isotope object both represents an isotope, but also a specific set of nuclear data representing that isotope (e.g., the 1001.80c ACE data). This is complicating things and muddying the water. Isotopes need to be sovereign and independent of the nuclear data (but probably really should represent an isomer). Then a nuclear data object (name TBD) can wrap that isotope inside a specific nuclear data library at a specific temperature.

This complicated my life because when adding metastable isomers to the Isotope object (#70) I wanted to add a function: def get_base_isotope(self): -> Isotope however with the nuclear data library information this function can have weird consequences and therefore has been tabled for the time being.

[MicahGale]

marked this issue as related to #14

[tjlaboss]

Should this also be separated from the concept of an isomer (#486)?

[MicahGale]

I'm not sure. With the branch mat_redesign this might be a null concern.