Don't suggest nat.flybrains

* results in a circular dependency
* just use it in tests if available
* Should really consider putting some very simple test data directly
  into this package rather than relying on external data
Streamline tests relying only on data in package
Merge branch 'fix/circdepends'

* fix/circdepends:
  Streamline tests relying only on data in package
  Don't suggest nat.flybrains
* closes #2