Add description of handling years in periods

[ClaraLuisa]

It would be great to have some explanation added to the messageix documentation on the handling of the years in periods. e.g. an explanation as given by @danielhuppmann in the hackathon chanell:

The period 700 includes the years [691, .. , 700] . Thus the duration_period_sum(year1, year2) parameter counts the number of years from the first year of period year1 to the first year of period year2
including the first year when counting. So duration_period_sum(689, 720) goes from January 1, 689 until December 31, 710, which is 22 years.

[khaeru]

Thanks to @DIUANA for flagging this in #385. For the record, I added a new documentation page describing year and period concepts, while finishing off #286. The current version is here. It probably could be expanded to describe these other parameters, or we could choose very precise and consistent wording in the tables that are inline with the GAMS code.

[DIUANA]

Many thanks @khaeru. It is important to have a clear documentation for the period related parameters. As I told in #385 it is necessary to align all these definitions. Atm I am in doubt if there is a mistake in the parameters definition or if I am misunderstanding them.