Add main ion fractions to the output

[anteplu]

In addition to the impurity ion densities for all impurity ion species available in the output, it would be great to have main ion species and main ion fractions too.

[jcitrin]

To achieve this I recommend the following:

1. Add main_ion_fractions to state.CoreProfiles analogous to impurity_fractions
2. Add main_ion_fractions to getters.Ions analogous to impurity_fractions
3. Whenever Ions is calculated ensure that the main_ion_fractions are assigned based on runtime_params
4. Ensure that CoreProfiles is updated/constructed with the main_ion_fractions from Ions
5. In output, ensure that the special shape of main_ion_fractions is correctly propagated to scalars
6. Include testing of the new output

[Mani212005]

Hello @jcitrin, can I try to add this feature ? Thank you.

[jcitrin]

Yes please go ahead. Note the recommended design above