Enable option to specify annual monthly range (fixes #95)

A single specified monthly range period should be considered annual and not seasonal. Also, in v0.15 the previous method of specifying a monthly range was lost due to a change in the format of clix-meta yaml files. This issue fixes the following:

1. A monthly range should be handled as an annual period, i.e. the output frequency should be yr instead of sem. Most of the logic previously in the Season class has been moved to the Annual class. (Season will need similar but different logic if/when custom seasonal specification is implemented.)
2. When using a monthly range, the output file will have two aux coords; year and season (previously season_year and season)
3. Monthly range is specified on climix command line as an argument to the annual period type, e.g. -p annual[fma]
4. The output file name will contain the monthly range together with the index name, e.g. fd-fma_*.nc