DataMgr:
	Make Get* methods const by making cache supporting elements mutable

NetCDFCollection:
	Remove deprecated staggered grid handling
	Add support for reading subsets of derived variables

VDC:
	File format should store the actual data range of each variable
	and DataMgr::GetDataRange should be changed to use this. Otherwise
	statistics used for, e.g. histograms, will be wrong

CurvlinearGrid:
	Broken with POP data in /glade/p/DASG/VAPOR/Data/POP/BryanASP/tenth_degree/daily

DC:
	periodicity should be a property of each coordinate variable, not 
	a data variable property. I.e. each coordinate variable should have
	a boolean flag indicating whether it is periodic or not. This would
	make more sense for unstructured grids
