disable compilation of the tests by default

they take half an eternity to compile and if you're working on an
external module which uses ewoms, this used to be a major time sink
which basically did not come with any benefit.

If you're interested in developing the eWoms core, pass
--enable-testing to the configure script or -DBUILD_TESTS to cmake...
set the last linarization to be non-reusable if a new epsiode begins

basically this is an violation of abstraction layers, as the time
manager is supposed only communicate with problem via a very limited
API and is thus supposed to know nothing about the model or the
assembler.

Anyway, this is changed for now as it was much too easy to get wrong
before...
assembler: rename dofDelta_ to dofError_

because the error is not the delta anymore...