Immobile domain porosity of zero causes divide by zero

[langevin-usgs]

Describe the bug
When using the GWT IST Package, it may be useful to mark some cells as having no immobile domain. A logical way to do this is to set the immobile domain fraction to zero, which sets the internal immobile domain porosity to zero. This can result in a divide by zero calculation in some cases. The code should be modified such that the mass transfer between the mobile and immobile domains is set to zero, and no further calculations are made. Should think about what to do with the immobile domain concentration in this case. Could leave it as the initial immobile domain concentration or mark it as a nodata value.