added a test for the behavior that shouldn't give nan. It currently fails. Now,
to make it pass.
decided the test should give 0 instead of 1. Fixes nan's returned.
applied katy's atom fraction nan patch with slight implementation tweaks, also included a guard in the mass fraction function
Merge branch 'atom-frac-bugfix' into develop

This fixes an issue with the compmap's massfraction and atomfraction interface for compmaps where isotopes exist but no quantity is applied. It will now correctly return 0 instead of NaN.