Implement occurrence terms for ReinsType=FAC

[johcarter]

Issue Description

We'd like to implement occurrence attachment and limit for facultative contracts, whereas currently we only support risk attachment and limit. This is because it is a more appropriate set of fields to use for Fac contracts and more in line with the labelling of financial terms in other platforms.

The concept of risk terms does not exist for Fac, because the whole contract applies to a particular risk.

The users will enter the occurrence terms in OccAttachment and OccLimit rather than in RiskAttachment and RiskLimit (not both) and the results will be identical assuming the fac contract only applies to one risk (=one record in ri scope file).

Occurrence terms for fac should be implemented in exactly the same way in Oasis as the other ReinsTypes. The terms apply at level 3 in the hierarchy in the input files.

Retain the use of RiskAttachment and RiskLimit in case of more complex / multi-risk contracts.

Example data / logs

Examples to be provided.