support other calendars

[jhamman]

The current time handling in FUSE is not likely to handle non-standard calendars. Recent discussions between the below listed folks have focused on properly reading the time units and calendar attributes from netCDF forcing files using these to calculate time vectors for any of the CF calendars.

Useful references:

• GARD implementation: https://github.com/NCAR/GARD/blob/master/src/timing/time_io.f90
• VIC implementation: https://github.com/UW-Hydro/VIC/blob/master/vic/drivers/shared_all/src/vic_time.c

cc @gutmann @naddor @nmizukami