Don't test SingleFromDoubleEdgeList and ParseEdgeList since part of graph-based parsing
Deprecate NrrdVoxDims, ReadNrrdHeader and remove is.nrrd

* also make .standardNrrdType a wrapper
Deprecate Read3DDensityFromNrrd and NrrdDataFiles
deprecate voxdims.gjdens
match.fun doesn't seem to work with a function specified as "package::FUN"
bring one read.neuron test up to date and retire another
update test for amira neuron with isolated points

* the old version was incorrect (e.g. using an isolated point as the start
  point makes little sense and more generally the fields in the main part of
  the neuron structure should all refer to the master tree - which unless
  specified will now always be the largest by number of points.
Inactivate test.igraphvsoriginal test for amira neurons

* basically committed to igraph approach at this point
* some neurons with loops are now unreadable but they were not being read
  correctly by the previous code