Simulators might modify the grid post EclipseState creation, so the Grid
fetched from there is unreliable. Copy the Deck-provided grid and apply
the manipulations at EclipseWriter construction time to ensure it uses
the same dimensions and has the same properties as the simulator.
Reading the start time from EclipseState means it's no longer required
as a parameter to every method, and that the current time can be
calculated as start time + time elapsed.
Hides all the EclipseWriter details and helper classes using pimpl,
which now enforces the non-copyability of EclipseWriter (which relies on
open and stateful file handles).
Rewrites the RFT test to be independent of opm-core data types and
functions, and renames it to test_RFT since the class now represents the
full file, not some open-closing handle.
