change construction of ParserKeyword objects to the factory paradigm

this makes it more explicit what the object is supposed to represent.
Merge pull request #239 from andlaus/ParserKeyword_to_factory

change construction of ParserKeyword objects to the factory paradigm
Fixed bug in extending TOPS vector.
Added EclipseGrid::getCellVolume() and getCellCenter()
Added Parser test of deck with TOPS keyword.