Merge branch 'feature/more-landmarks'

* closes https://github.com/jefferis/nat/issues/192
* feature/more-landmarks: (22 commits)
  More examples for landmarks i/o
  No longer export *.landmarks.amira
  No longer export *.landmarks.cmtk
  Remove Force parameter from write.landmarks.cmtk
  fix some buglets in write.landmarks
  Add generic write.landmarks function
  *.cmtklandmarks->*.landmarks.cmtk
  test generic read.landmarks
  Add generic read.landmarks function
  Register cmtklandmarks format
  Function to identify cmtk.landmarks files
  rename *.amiralandmarks -> *.landmarks.amira
  register fijilandmarks format
  Add is.fijilandmarks
  remove unused read.landmarks.fiji Verbose arg
  test reading file with just 1 landmark
  specify tolerance
  test reading fiji landmarks
  don't name the rownames for fiji landmarks
  rename landmarks test file
  ...