Verbatim copy of plotneuron3d.simple
Clean implementation of plot3d.neuron

* removes NeuronList and HighlightLongestSegment args
* examples (but no tests)
examples for cmtk.bindir()
document cmtk.bindir() @return
Now cmtk.bindir(check=TRUE) makes sure it is not null

* also squash warning if which gregxform fails
options(nat.cmtk.bindir=NA) insists that cmtk is not installed
use cmtk.bindir() to find cmtk command line tools whenever they are used
don't test cmtk command line tools when cmtk is not installed
fix cmtk.bindir when checking path
don't show output of which gregxform in console
check/set CMTK location at package startup