fix spurious GCC warnings

The root cause are pretty (too?) strict flags w.r.t. compiler
warnings. In this case, the offenders are "-Wmissing-declarations" and
"-Wtype-limits".
Merge pull request #227 from andlaus/fix_gcc_warnings

fix spurious GCC warnings
Added Box() class to book keep the indices in a box.
Added constructor for sub box.
Added Box::equal() comparison function.
Added BoxManager() to keep track of the currently active input box.
ERT-656: Changed GridProperty to take (nx,ny,nz) instead of volume.
Basic BOX support when reading integer properties.
Added iterators begin() and end() to DeckKeyword class.
Added copyFrom() property to GridProperty
EclipseState now handles COPY keyword.
Added support for MULTIPLY keyword.
Added new keyword PVTNUM
Supporting the ADD and EQUALS keywords.
Added name() property to section objects.
Added double properties for e.g. PERM and PORO to the EclipseState
Added const qualifier to EclipseState::getxxxProperty() methods.
Renamed getXXXProperty -> getXXXGridProperty
Avoid use of {} in init to satisfy clang.
Using size_t instead of int for EclipseGrid::getN?