Split check_emission_intensity into check_prep_emission_intensity and check_plot_emission_intensity

[jdhoffa]

Also requires adding additional checks to both functions, since it is expected that their inputs should differ dramatically (so likely the checks should too)

          I agree in principle that the `checks` likely should be different as they test different input. However in practice, the were checking the presence of the exact same columns. 

That to me suggests that check_plot_emission_intensity and check_prep_emission_intensity likely should be expanded to include more precise checking, but I think that is out of scope for this PR.

I will open a new issue to that effect.

Originally posted by @jdhoffa in #543 (comment)