Dealing with extra source columns

[andrus]

Per #59 we allowed LinkMove to include extra source columns that are not directly mapped to anything in the target. We broke our convention for target naming though. Now transformation attribute "target" can be an arbitrary String, not resolvable against the target entity.

To avoid this, we may need some redesign:

• do not propagate extra values into target-keyed maps
• fail on invalid attribute target expressions in extractor XML

We should also check how this works in CSV extractor ... CSV extractor currently has null source names, so we need to give users a way to address such extra columns