FeatureFinderIdentification: documentation, tests, parameter improvements

What the title says. Note that only FeatureFinderIdentification is affected by this pull request (no changes to other classes), so merging it should hopefully be uncontroversial.

A lot of intermediate commits included here were about the addition of a feature that was later removed again (https://github.com/hendrikweisser/OpenMS/commit/553c95a1f20eed721f9905e08e25d596f5778359). It seemed simpler to remove the code for that feature than to unravel the relevant commits.

The test case is subject to change in connection with issue #1060.
I'm about to test it. There's also a workaround for missing protein accessions in lines 854-858 (https://github.com/hendrikweisser/OpenMS/blob/FFId/src/topp/FeatureFinderIdentification.cpp#L854) - which I can't seem to directly annotate here for some reason. Maybe consider handling that as well?