convert array of netCDF4.datetime objects to numpy.datetime64 array to support virtual_variable indexing, #121
restrict the conversion of netCDF4.datetime objects to numpy.datetime64 object to valid years (1677<time<2262)
Handle arbitrary shape time arrays in nctime_to_nptime.  Also added the ability to fallback to netCDF4.datetime objects since this solution does not fix the problem for all calendars in all situations.
cleanup fallback code a bit
Add further testing for single element time ordinals.
Expanded warnings for when non numpy.datetime64 arrays are returned.
Use context manager for warnings in test_conventions.py.
filter warnings from earlier tests
correct filter message for warnings
add test to catch fallback warnings, insure returning a numpy array during fallback of single element
Merge pull request #126 from jhamman/cast_nc_datetime

Return numpy.datetime64 arrays for non-standard calendars