Add the ALL keyword (v2)

this requires some changes to the test code generator: the problem was
that the name of the test for the ALL keyword caused a naming conflict with
Opm::ParserItemSizeEnum::ALL. I haven't considered the possibility to
break the build by just adding a new keyword, so sorry for the breakage.

On a more philosophical ground, I think it is a bit questionable to
auto-generate test cases at build time because they will automatically
match the keyword definition, whether this definition makes sense or
not...