List netCDF4 as a pygmt dependency

xarray needs the netCDF4 package for reading (perhaps also writing) netCDF files.
However, netCDF4 isn't installed by default since it's an optional
dependency of xarray. Although pygmt doesn't import netCDF4 package
explicitely, some features won't work without the package.

See #239 for past discussions.

Author: seisman

doc/install.rst

@@ -55,6 +55,7 @@ PyGMT requires the following libraries:
 * `numpy <http://www.numpy.org/>`__
 * `pandas <https://pandas.pydata.org/>`__
 * `xarray <http://xarray.pydata.org/>`__
+* `netCDF4 <https://github.com/Unidata/netcdf4-python>`__
 * `packaging <https://pypi.org/project/packaging/>`__
 
 The following are optional (but recommended) dependencies: