NetCDF is a popular file format in environmental forecasting to provide
large amounts of (ensemble) data in binary format. The NetCDF4
specification uses HDF5 under the hood, enforcing a simplified layout.
Delft-FEWS also support NetCDF(-CF), and reading/writing it can be much
faster than PI-XML.

This commit adds support for NetCDF files. The data format is provided
in data/netcdf.py, just like CSV and PI support. On top of this data
wrapper, there is a NetCDFMixin implementing the read/write methods of
the generic IOMixin base class.

There are specific requirements to the NetCDF (.nc) files to match
Delft-FEWS (and the NetCDF-CF standard):

- The station identifier variable in the .nc file should have a
  "cf_role" attribute with value "timeseries_id"
- The time variable should have an axis attribute with value "T" and
  "standard_name" attribute with value "time"
- Members in an ensemble are indexed with coordinate variable
  "realization".

Note that when only a single ensemble member is to be read the use of
"realization" is optional.

NetCDF import has been added as _optional_ functionality. We are not yet
sure if we want to force all users to install all dependencies for all
(future) IO formats, even if they do not need them. As it stands, "pip
install rtc-tools" will therefore not install the netCDF4 requirement.
Instead, one can either install the "netCDF4" dependency manually, or
use "pip install rtc-tools[netcdf]" or "pip install rtc-tools[all]".

Closes #1007