refactor out mirror registrations
split brain2reg into mirroring and bridging

* since there is no common code as written
factor out code to find registrations

* still need to figure out how to handle extdata
Switch to single option (regdirs) for all registrations

* this will include packages and user directories
switch to use of single regdirs option

* functions and tests
Merge branch 'feature/op.regdirs'

* feature/op.regdirs:
  switch to use of single regdirs option
  Switch to single option (regdirs) for all registrations
* closes #3
remove redundant extdata function

* now we just use options('nat.templatebrains.regdirs') for everything
* closes #1
give find_reg a mustWork option

* and allow this to be passed through mirror_reg and bridging_reg
Detailed tests for finding registrations