Fix PORV tests v2

next try to fix the PORV test of #329. the problem was uglier than I expected, see the commit message of the second patch. On my machine, it fixes the segfaults for GCC 4.4 in debug and release mode using boost 1.44 and Clang 3.5 with boost 1.54 in debug mode. other configurations will probably work too, but I didn't test them...

One issue remains: on GCC 4.4 with optimizations, I get test failures of the following kind:

```
70: unknown location(0): fatal error in "PORV_cartesianDeck": std::logic_error: Only <double> and can be meaningfully queried for nan
70: /home/and/ctest/gcc-44/src/opm-parser/opm/parser/eclipse/EclipseState/Grid/tests/PORVTests.cpp(232): last checkpoint
```

these seem to be unrelated, though.