remove template-brains alias from package docs
move extdata function to nat.templatebrain
remove redundant NAMESPACE entries

* looks like roxygen didn't run properly ...
import nat.templatebrains to keep CHECK happy
remove redundant package dependencies

* there is no actual need for either of these in this package though nat would
  almost certainly be used to do load/plot transformed image data. One could
  make nat a suggestion, perhaps for example code, but that is probably as 
  much as is required.
* this also fixes a CHECK note about not importing from “depends” packages