make cmtk.targetvolume generic

* and split off docs from cmtk.reformatx
cmtk.targetvolume should work for im3d/templatebrain

* anything with an as.im3d method should be fine
* could also make a cmtk.targetvolume.templatebrain method if desired
test for im3d functionality of cmtk.targetvolume

* could add a test in nat.templatebrains as well
roxygenise
docs: tidy up cmtk.reformatx
Merge branch 'feature/generic-cmtk.reformatx'

* feature/generic-cmtk.reformatx:
  docs: tidy up cmtk.reformatx
  roxygenise
  test for im3d functionality of cmtk.targetvolume
  cmtk.targetvolume should work for im3d/templatebrain
  make cmtk.targetvolume generic
* closes #147