Empty ABUNMOLn columns written to grid file if user supplies molNumDensity()

It would be better to write columns from the `nmol` field of the grid point struct, since this is always calculated, either directly from the user's supplied `molNumDensity()` function, or indirectly via `abundance()`, `density()` and `par->nMolWeights` etc. The `abun` field of the grid point struct in contrast can be left unfilled in some circumstances.