RFC: add class for simple tables

This is not yet the final version which should be merged, but I would solicit some feedback. The class intended for keywords like SWOF which currently is a big vector of doubles but one needs to use the data in a column  oriented way. Also, I've put the code into opm/parser/eclipse/EclipseState for now, because from the logical POV this belongs to the "mythical fourth layer" even though its probably not a state class...

This class is intended to be used like this:

```c++
Opm::DeckKeywordConstPtr swofKeyword = newParserDeck->getKeyword("SWOF");
Opm::SimpleTable swofTable(swofKeyword,
                           /*columns=*/std::vector<std::string>{"SW", "KRW", "KROW", "PCOW"},
                           /*recordIdx=*/table_num);
const std::vector<double>& sw = swofTable.getColumn(0);
// ...
```

what would be really useful is to move the column names into the JSON description of the keywords, but I could not find a way to go back to the parser from the deck/keyword. maybe this is not even desirable as decks might also be created by other means...