- provide generic methods that delegate to `GapObj_internal`
- add tests
- move an `end` of a `@testset` to a better place

- change the `julia_to_gap` methods such that `GapObj_internal`
  gets called with the same `recursive` value;
  this way, we get the same behaviour for `julia_to_gap` as before
  the switch to `GapObj_internal`,
- extend the tests of the backwards compatibility to the conversions
  promised in the documentation,
- fix the conversion rule for matrices:
  call the conversion of the row objects with the given `recursive` value
  in order to convert the matrix entries or not, as requested,
- and use the term `FFE` in the documentation, not `GapFFE`