Flybrains

* Rename IVLP neuropil to WED in accordance with Ito et al 2014 (37)
* Doc tidying and fixes

* add D. sechellia DsecI template and surface model (with Richard Benton)

* add information and surfaces for Drosophila melanogaster, virilis and
simulans template brains
* add DOIs to template brain information
* remove functions to support download/update of addition external registrations
as these are now in nat.templatebrains v0.6
* add function to download all Jefferis Lab registrations

* added functions to support download/update of additional external
registrations from github repositories (see ?add_reg_repo and
?update_reg_repos)
* Add any additional registrations to options nat.templatebrains.regdirs on
startup
* add VNCIS1 template brain and surface model

* package loading adds appropriate directories to the option
nat.templatebrains.regdirs.
* dev: add real tests for via registrations
* dev: reinstate detailed transformation tests

* add T1 mirroring registration
* move tests to nat.templatebrains as well. We may yet want to streamline the
tests in nat.templatebrains and move some of the more detailed ones back
here.
* depend on version 0.2 of nat.templatebrains