make all headers includable without preconditions

There is a hack required for BlackoilCo2PVT so that it does not
conflict with BlackoilPVT...
remove the SimpleRock class

it has been unused...
remove the SintefLegacyGridInterface class

because this one also has been unused
remove the EulerSolverTester class and its only "user" in attic/

this class was unused (and was probably bitrotten because of this).