to add VTK format to nrrd:

nrrdEnums.h:
added entries to nrrdFormat* and nrrdMagic* enums

nrrdDefines.h:
added #define NRRD_EXT_VTK    ".vtk"

arraysNrrd.c:
augmented nrrdFormatIsAvailable[], nrrdFormatIsImage[],
  and _nrrdFormatUsesDIO[]

enumsNrrd.c:
augmented _nrrdFormatStr[], _nrrdFormatStrEqv[], _nrrdFormatValEqv[],
  and _nrrdMagicStr[]

simple.c:
augmented nrrdFitsInFormat()

read.c/write.c:
wrote reader and writer

(also augmented usage info in unu save)
