make netcdf4 an optional dependency

## Description

One of the processes `nc_meta` needs the `netcdf4` library. Not a big deal currently in Emu ... but we could make this an optional dependency to have an example how to handle this.

## Environment

* Emu version used, if any: 0.9
* Python version, if any:
* Operating System:

## Steps to Reproduce


## Additional Information

