unit vs units in netCDF output

• ncdump -h of mHM's netCDF output shows that the name used for unit/units are not uniform.
• The word units is used for the attribute of grid variables. This is usually the case/ found elsewhere.
• The word unit is used for the attribute of mHM variables.

This is minor issue. However, ratifying this would reduce some lines of post processing of mHM (e.g. in forecasting chains).