astemf (and other) parameters should have nonzero missing value

As part of my crop reparameterization work, I noticed that the `astemf` parameter is set to the missing value for some crops. This would have caused errors (at least in debug and/or nag) in some comparisons, so I was confused until I realized that the missing value (or fill value) for that parameter is zero. This should be changed to avoid confusion when inspecting the paramfile—the missing value should be something truly meaningless, and non-crops should be set to that.

This may be true for other crop allocation parameters as well.

Adding "next" to discuss milestone.