Add support for wave model variables

### What problem does your feature request solve?

To date CSET has been designed and used with focus on atmosphere outputs.
There is value in ability to support variables from other model types (e.g. as implemented in ukep_plot to support consistent regional coupled evaluation across model components).

### Describe the solution you'd like

Introduce support for WaveWatchIII model variables.

WWIII outputs are netCDF, and easily read in by iris.load.
Therefore this development only needs to cater for introduction of wave-specific diagnostic naming and default colorbars etc.

Proposal might be to introduce new _colorbar_definition_wave.json template rather than further extend the default?

### Describe alternatives you've considered