Add cell.is_mass_dens

[MicahGale]

Is your feature request related to a problem? Please describe.

I was trying to free-hand code and I first attempted cell.is_mass_dens instead of the correct, cell.is_atom_dens. Also sometimes for me saying not cell.is_atom_dens takes me an extra second to comprehend.

Describe the solution you'd like

I'd like to add the simple propery:

class Cell:
    ...

    @propety
    def is_mass_dens(self):
         return not self.is_atom_dens

Describe alternatives you've considered

Keep the status quo.