create_context_ids - edit required

[ehwenk]

I've discovered a tweak required for create_context_ids - if there are NA's in the context column, the column is assigned an integer that doesn't show up in the contexts table. For instance in Choat_2006, for population-level measurements, the temporal context columns are NA, but those entries are assigned temporal_id = 11 . So somewhere around line 496 in process.R we need to only apply the function to columns where they aren't all NA. I'm thinking the best option is to create a T/F vector of which columns aren't all NA and then only applying the function to the True's?