NetCDF: Fix conversion of imported times to datetimes

NetCDF4 depends on cftime, the latter released a version 1.1 which changed the behavior of the num2date function. It also added a new convenience method num2pydate that we can now use instead.

We only implicitly depend on cftime via NetCDF4, but with this breaking change on their end we need to make sure that the correct version is installed. We therefore add cftime to our list of (optional) dependencies.

See e.g. https://github.com/Unidata/cftime/issues/134 for details on why this breaking change was made.