Annual single month (fixes #266)

Joakim Löwrequested to merge
joakim.low/climix:annual-single-month into main

(Based on branch in MR !193 (merged), keeping as draft)

Added support for single month range in annual period. Is specified by first three letters of a month on command line, e.g. -p annual[feb].

Also added some additional text to climix command line help.

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