Accommodate multiple exposure types in db backend #90

Various reserving methods may use some kind of exposure measure, for example, the expected loss method often involves claims per unit of exposure:

The challenge for FASLR is to anticipate any kind of exposure and to store it in the backend. Thus, having predefined types and only allowing those will do no good if a user has a type of exposure we didn't anticipate. Therefore, we need to be able to, either by extensions or some clever way of engineering the db, to accommodate multiple exposure types.

I'm thinking maybe something like: