176 Variables definition from expression

[evenmn]

Added support for:

• defining variables from expressions
• ws is automatically converted to the expression "([10u]**2+[10v]**2)**0.5"

Variables expressions support basic math operators, and expression mode is invkoed if there is a left braket '[' in the variable definition:

Example:

- verif:
          filename: /path/to/output.nc
          variable: ws
          units: m/s
          obs_sources:
            - anemoidataset:
                dataset: ${verif_dataset}
                variable: "([10u]**2+[10v]**2)**0.5"

This functionality was also incorporated into the NetCDF output class, here demonstrated by converting total cloud coverage (tcc) from the range 0-1 to 0-100:

- netcdf:
    filename_pattern: /path/to/output.nc
    variables: [2t, msl]
    extra_variables: ["100*[tcc]"]

[havardhhaugen]

kan du rebase?

[havardhhaugen]

Sorry can you rebase again? I tried doing it but it was slightly complicated so I'm afraid to mess things up.