Explore speed increase in deck parsing

If https://github.com/OPM/opm-common/issues/2262 is solved, explore how this can be used in ecl2df's deck parsing module in order to not parse non-interesting parts of the deck. Typical usage would be to ignore the entire GRID section when saturation function are requested.

Start by reading OPM/opm-common#2262, then inspect ecl2df's deck parsing module and how saturation-function requests identify the needed sections. Done means the parser can avoid irrelevant sections such as GRID for that request and demonstrates a measurable speed improvement.

Written by the indexing model from the issue text.