fix breaks in histogram objects generated by ReadHistogramFromNrrd

* unu's axis min and max seem to me to match the midpoints – at least for integer nrrds
  think about this for floats
Avoid explicitly finding min and max (i.e. range) for 8 bit nrrds
Deprecate DropMissingFactorLevels

* droplevels has been in R base since 2.12
Reorganise neuron/neuronlist code with a view to deprecating Cell2007-specific code

* would like to move core neuron/neuronlist code into a package sooner rather than later
Add documentation from remaining neuronlist functions
Fix paths to test files for ReadNeuronsFromLongairTraces
Fix a column name error in CMTKStatistics test

* I renamed the first column to something more sensible
  (but forgot to change the test)
Add runitInterpolateAlongNeuron.R