Verbatim copy of ReadNrrdHeader & .standardNrrdType
normalise read.nrrd.header
Add is.nrrd
tests for is.nrrd
basic test for read.nrrd.header
Verbatim copy of Read3DDensityFromNrrd
normalise Read3DDensityFromNrrd as read.nrrd
fox NrrdDataFiles
doc fixes
verbatim copy of NrrdVoxDims
Fix up nrrd.voxdims
Add test for read.nrrd
roxygenise docs/NAMESPACE for new functions
move .standardNrrdType
NrrdDataFiles -> nrrd.datafiles

* still not exported
export is.nrrd
fix handling of missing origin

* now it is null rather than missing
verbatim copy of Write3DDensityToNrrd
Write3DDensityToNrrd -> write.nrrd

* and other normalisation
fixes noticed by check & export write.nrrd