add is.templatebrain

* and test
docs for is.templatebrain
Teach bridging_reg to handle character input and checkboth
as.character.templatebrain
fix as.character.templatebrain docs
test as.character.templatebrain
simplify bridging_reg using as.character
bridging_reg adds a swapped attribute when required

* this can be used to set the direction later
* includes test
deactivate numerical inversion warning

* I'm not sure that it's accurate since when streamxform is given a registration
  template_sample.list that maps a sample image to a template image, it actually
  has to invert the registration in order to transform points sample -> template
refactor xform_brain to use smarter bridging_reg
refactor to use as.character

* means it can accept brain or character specification
fix: checking number of via registrations