Use h5py or xarray for Cube generation

[forman]

We currently use the netCDF4 library for opening source files which seems to have problems opening netCDF files > 6GB which is a no-go for our Air Temperature products.

We may switch to h5py or use xarray with the h5netcd driver to get around these problems.

See #29

[gunbra32]

The error is known and there seems to be a fix, which apparently hasn't made it through to the conda distribution.
Unidata/netcdf-c#188

[gunbra32]

We stick to netcdf for the time being. Downloaded source data in monthly files with two time steps per day.

[forman]

Not an issue