Remove using time var when doing just a spatial subset, and don't require it to be in the output subset file

[jamesfwood]

Spatial subsets shouldn't use or require a time var. Also, for some collections temporal subsetting should not be an option. Investigate when can we actually do a temporal subset, and when it doesn't make sense to allow it. Only require time var if doing a temporal subset.

Talk with Jack about this as well. Maybe we will just completely remove the option for temporal subsetting to make things a lot easier and simpler. I think I remember talking with him about this in the past and he said we shouldn't be doing temporal subsetting in L2.