After removing initial settings of history fields to spval, search code for remaining uses of spval

[billsacks]

After doing #1347 we should search for remaining uses of spval in the code, making sure the only ones left really should be there. In particular, we should make sure we don't have any more uses of spval to represent uncalculated values in certain landunits. This is important to make sure the landunit_mask metadata is correct, and for some further cleanup that we can do if spvals are no longer used in this way.

[billsacks]

I see initialization of the output array to spval in p2c_1d (and maybe other subgridAveMod routines). I think this could actually abort if a given output array value would end up unassigned. Or, if that doesn't work, then could initialize to NaN rather than spval.

[billsacks]

Blocks #1349