Add '/usr/local/lib/cmtk/bin' to dirs to check for cmtk

* recent versions seem to install a cmtk script to /usr/local/bin
  and then put all the binaries under /usr/local/lib/
teach xyzmatrix to handle data.frames and matrices with >=3 cols

* 3 cols are assumed to be points regardless of names
* >=3 must have names
split off xyzmatrix methods for neurons and dotprops objects

* dispatch keeps functions simpler, but still keeping
  is.neuron check in xyzmatrix.default to handle older neurons without class