[Opm] Renaming module: opm-autodiff -> opm-simulators

[Atgeirr Rasmussen]

Dear OPM community,

The opm-autodiff module contain quite a few simulator programs, including the Flow family.
Soon there will also be some simulator programs there that are not based on AD (moved from
opm-core).

This renaming is intended to make that clearer, and make it easier for users to find the tools
they want to use.

We will do this like before: rename repository, create PRs for affected modules, inform via the
mailing list.

Atgeirr