Add time spanned incapacity for work

[mischa-s]

-incapacity_for_employment__corporation_determination does not need to be surfaced in the interface

• instead it should be calculated using two variables, incapacity_for_work and incapacity_caused_by_covered_injury which should both take multiple time spans as inputs and convert these to true false sets for OF
• if the given day is in the time spans for both of the new variables, then incapacity_for_employment__corporation_determination should resolve as true