Allow marking expressions to be defined as a column at first use

The implementation is straightforward: adding a forwarding node to identify (parts of) expressions that need to be defined, an reusing the existing code for the rest. The user-facing part is op.defineOnFirstUse which returns the wrapped expression result.

I actually think this should be the default for get_scalefactor (the definition will basically always end up in the right place for those: after the Filter (if any) and before the weight definition)... any opinions?