Command line option to specify computational period

Add a command line option to specify the period to perform index calculation on. Reasons:

1. User may only want to compute the index for a sub period of the data.
2. Beginning and end of input data may be incomplete with respect to time period. E.g. if 'annual' and first and/or last year of data does not contain entries for all days of year, climix may raise error for some indices.

See also #273 (closed) and #257 (closed)