Gap sheets integration

Added gap sheets, along with tests for the gap sheet calculations.

@csytan We should talk about this when we've in the same place next. The numerator / denominator splitting adds a bunch of complication to the gap sheets that may as well be avoided. Also, we should pull this data into NMIS and see if this is the data that is required; I know that the gap sheet names are a bit special in NMIS.

@myf I didn't populate output_indicators for gap sheets. They were enough of a pain as is. We could make an issue to do it in the future, and would appreciate your help there.