Add ability to query units from netcdf.

The time units and calendar give you a start time, time step size and
calendar which you can use to interpret what the timestamps mean.
Units on the other variables are likewise a good thing to have access
to.

Change-Id: Iebb151973e53fee120f5d3874aaa3d95401b395b
Merge topic 'netcdf_units' into master

d39346dd Add ability to query units from netcdf.