process_custom_climate_data inefficiently reads all data to disk

[fmaussion]

process_custom_climate_data is highly inefficiently reading all data to disk before subsetting. This is happening here:

oggm/oggm/core/climate.py

Lines 119 to 122 in 154c01e

	temp = nc_ts.get_vardata('temp')
	prcp = nc_ts.get_vardata('prcp')
	hgt = nc_ts.get_vardata('hgt')
	ttemp = temp[:, ilat-1:ilat+2, ilon-1:ilon+2]

This is really not how this should be done (subsetting should happen before the read)