Added new EclipseGrid constructor

The new EclipseGrid constructor EclipseGrid(nx,ny,nz , GridSection)
allows the construction of a GRID without the RUNSPEC section.
Merge pull request #280 from joakim-hove/EclipseGridConstructor

Added new EclipseGrid constructor
Add dimension (length) to THRESHOLD_THICKNESS item.
Also add dimension to MAX_EMPTY_GAP property.
Merge pull request #281 from atgeirr/pinch-threshold-dimension

Add dimension (length) to THRESHOLD_THICKNESS item.
Add isPinchActive() and getPinchThresholdThickness() members.

Now reading the optional PINCH keyword from the GRID section.
Add test for pinch features of EclipseGrid.
Make getPinchThresholdThickness() throw if isPinchActive() is false.
More tests for proper pinch-related behaviour.

Now also check behaviour for no-pinch case.
configure: also set the CMAKE_C*_FLAGS_DEBUG and CMAKE_C*_FLAGS_RELEASE of CXX_FLAGS is specified

for me, setting only CMAKE_CXX_FLAGS did not do the trick when I was
trying to compile everyting with libstdc++'s debug mode enabled. (it
caused linker errors because opm-parser was not linked against
std::debug in this case.) Maybe I got something wrong, though...
Follow opm-parser naming conventions for data members.
Name magic constant (invalidThickness).
Merge pull request #282 from atgeirr/pinch-eclipsegrid

Add pinch features to EclipseGrid class
Merge pull request #279 from andlaus/set_more_flag_variables

configure: also set the CMAKE_C*_FLAGS_DEBUG and CMAKE_C*_FLAGS_RELEASE ...
Add new constructor taking a Deck.

The new constructor allows us to create EclipseGrid objects from decks
that do not have a strict sectioned structure.

Also modify some implementation details. Many methods are now templates,
and can take Deck or Section objects (they have the same
hasKeyword/getKeyword interface).
Allow DIMENS to be used as well as SPECGRID when no sections are found.

Also refactor DIMENS/SPECGRID to dimension code as a function.
Added test case for construction from deck with no sections.
Create pointer-to-const objects.

Older gcc (4.4) does not allow conversion of shared_ptr<Foo> to
shared_ptr<const Foo>. Pointed out by @bska.
Remove section-using EclipseGrid constructors.

Also:
 - Simplify implementation (no templates).
 - Update tests.
relax strictness of the section handling and fix/work around a few bugs

We now do not require the sections to be correctly ordered and the
presence of the mandatory sections since even the unit tests did not
always specify all mandatory sections, which lead to a section
containing the rest of the deck if one of the expected next sections
was not specified.

also it seems like the DeckKeyword::getDeckIndex() does not correctly
work in some situations which lead

assert(deck->getKeyword(i)->name() == startKeyword);

to fail in the Section::populateKeywords() method. now the deck is
always sequentially traversed to find the position of a section's
start keyword. (This is necessary anyway if one wants to make sure
that the deck does not specify the same section more than once, a
feature which this patch also adds.)