rename adios2_engine_type -> adios2_io_engine_type

Created by: germasch

This is an API change (in the C bindings), but I believe adios2_engine_type right now is exclusively used by the Fortran bindings and one test (that I had added), so it's very unlikely that any external user is affected.

Its Fortran (f2c) version is already named that way, and it makes more sense, since the function actually takes an adios2_io, not an adios2_engine. This will allow to later add a adios2_engine_type function to the C bindings that actually takes an adios2_engine (if needed). (The CXX11 interface already has Engine::Type(), which does not have an equivalent in the C or Fortran bindings).

I also changed the argument order to take the adios2_io first, since most other adios2_io binding functions do so.