Bring back detailed transformation tests

* but note that these don't work with devtools test() because there is a
  call to system.file in nat.templatebrains::brain2reg that devtools
  cannot override.
* so brain2reg is looking for stuff in nat.flybrains/extdata when it
  is still in nat.flybrains/inst/extdata
Add .onLoad to set nat.templatebrains.regdirs

* this provides a simple mechanism for finding registrations