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
Move EulerSolverTester to attic.

Its only client is also in the attic.
Merge pull request #113 from atgeirr/fix-headers2

Make headers includable (revised)