Parse compositional summary keywords

[goncalvesmachadoc]

I can run OPM/opm-tests#959 using this implementation.
It parses: "WXMF", "WYMF", "WZMF", "WCGMR", "WCOMR" , but does not output anything.

[goncalvesmachadoc]

jenkins build this please

[bska]

This is a good start, but as you say won't actually internalise any of the nodes or output any values to the summary file. Are those tasks intended as follow-up work here, or are we aiming for a minimum support level of "don't stop the run if we encounter these summary keywords"?

[goncalvesmachadoc]

This PR intention is just to avoid stopping the run for the benchmark cases.

[bska]

With the knowledge that this is primarily a stop-gap measure to avoid crashing when running cases featuring these summary keywords I'll merge this. We'll develop actual support for calculating and outputting the values to the summary file as need arises.