Change contex of 'model' to 'mapper' in join rules [SERINA-10]

There seemed to be no reason to specify a model, when you could specify
a mapper instead, and skip some concatenations which looked really
annoying. This is pretty cosmetic really.
prepare() now stores the output instead of returning [SERINA-10]

This allows prepare() to be used as a query "anchor" and allows you to
chain more easily
Use getMapperNameFromModel() [SERINA-10]