tests for new neuronlist-dataframe methods
Don't run example resetting cmtk.bindir()

* this was causing trouble after using devtools::run_examples
Fix missing alias for droplevels.neuronlist
verbatim copy of subset.neuronlist from AnalysisSuite
Clean up implementation of subset.neuronlist

* But ... passes test(), but fails check()
Fix bug in finding functions that are not in current frame

* now passes check
add example of including global variable in subset expression

* for some reason (presumably due to the frame in which the subset call is
  evaluated) this fails as a test (which I have inactivated) but works fine
  as an example (which I have left active).
Prepare v0.5.2