source indices should use numpy integer type from constants #250

Closes #250.
360_day calendar breaking non-NetCDF outputs

If conversion to pure Python datetime objects fails, then the netcdftime
datetime object is used. This required removing microsecond which is not
supported by netcdftime. There could be issues associated with the occasional
netcdftime object, but tests are not catching any at the moment. Issues
may result from uniquely placed time subsetting and conversion.

Closes #251.
Removed ocgis metadata from source metadata output. This required
creating another connection. This information could be retrieved
by using the standard Inspect and generated unnecessary overhead
for most requests.
unit management

Unit validation is now allowed at the calculation level. The
`cfunits` property of `Variable` was changed to allow `None` units
(i.e. dimensionless). Doc changes included.

Closes #183.
Merge branch 'master' into i238-icclim
mark - very minor change to imports