Port Foam::Time::setDeltaT to new signature

[MakisH]

On February 5, 2018, this commit to openfoam-dev broke the compatibility with the OpenFOAM adapter, as the method Foam::Time::setDeltaT now uses only one argument instead of two, changing a bit its logic.

We need to find a way to support both the old and the new signature and to check how the logic changes affect the adapter.

[MakisH]

This is treated in #33, will not be merged anytime soon (but treated in #32 )