rename 'time_units' and 'start_time' attributes of time coordinate to 'units' and 'start'

[OnnoEbbens]

It seems easy but the 'units' attribute of the time coordinate seams to be locked and I get this error when I try to write it to a netcdf file: ValueError: failed to prevent overwriting existing key units in attrs on variable 'time'. This is probably an encoding field used by xarray to describe how a variable is serialized. To proceed, remove this key from the variable's attributes manually. It is also mentioned here: pydata/xarray#3739. I don't have time now to dive into this. so I will revert the commit for now and create an issue for this.

Originally posted by @OnnoEbbens in #58 (comment)