Fix bug resetting mdatoms masses to lambda=0 state

Fix bug resetting mdatoms masses to lambda=0 state

When DD is used, the mdatoms masses were reset to their lambda=0 state
after every DD step. This is fixed by making the DomDecHelper aware of
the FEP element.

Note that the use of MDAtoms should likely be revisited to allow for a
more elegant solution, but this would likely require changes which are
less local than the proposed solution here. Further development is
therefore deferred to #3700.