Streamline Conditions Derivation

Closes #45 (closed).

Modify addConditionDerivation so that it now takes an input condition key (or keys), an output condition key, and a callable which will be invoked with the objects corresponding to the input keys, and which returns an object corresponding to the output key.

The number of input keys must match the number of arguments of the callable, and the retrieved condition must be convertible to the type of the corresponding argument.

to be applied in conjunction with Rec!1658 (merged)