make the getSIDouble*() methods const

the fact that data is lazily converted to SI units should not be
relevant for the caller. This requires to decorate the array for SI
units in the DoubleItem as mutable, though...
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"));
Merge pull request #81 from andlaus/decorations

add some method decorations