Neutral pump should raise an exception if is_pump not present in grid file

[mikekryjak]

This would help prevent the stupid mistake where you forget to put the field in and run a case for a long time wondering why the density keeps increasing. This just happened to me for the third time.

Edit: the field was actually in the grid file, but because I mistakenly wrote it as a Field3D instead of Field2D, it was ignored by BOUT++ upon reading.