More user friendly application of choice of period (monthly, seasonal, annual, user specific...)

Right know the period on which the index is calculated is to be set in the yaml file

    period:
      allowed:
        annual:
        monthly:
        seasonal: 
      default: seasonal

and the default value is taken.

In case, like for klimatfabriken, one needs to calculate the index for multiple periods, it is very very error prone to have to change the default value in the yaml file, which lies somewhere long in the climix tree.

I think it is very necessary to add period to the parsed arguments from the command line, or find another solution.