add a few 'explicit' statements

this allows to constructs like

ParserIntItem("NRPVT");

which IMHO is more readable than the previous way, i.e.,

ParserIntItem(std::string("NRPVT"));
add class for simple tables

This is 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.

This class is intended to be used like this:

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 might be 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 desireable as
decks might also be created by other means...