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.
Don't think the "Tenor" in the `toString` adds any value now. Merge `toFormattedString` into `toString`.