REQS-382 - Add in Tenor class to basic finance

This adds in a Tenor class to the basic finance module. This is a simplification of what is in the current platform and removes:
 - BusinessDayTenor, as this will be replaced by offsetBusinessDays in curve construction
 - Comparable, as it makes no sense to compare tenors without some form of valuation date acting as a reference point.
I'd prefer to see all static factory methods at the top before the constructor. In general, "of", then "from" then "parse", then constructor, then simple getters, then functionality.