Lost interface documentation of native and symplectic modules

[mseri]

It looks like the changes to owl_ode.mli introduced in https://github.com/owlbarn/owl_ode/pull/14 have made the part of documentation of the Native and Symplectic modules disappear (not the autocompletion or merlin information, just odoc's documentation).

This may be a chance to document better the interface in a new native.mli and symplectic.mli module, although it would be nice to be able to re-enable odoc documentation there