Migrating WELSPECS part of WellsManager constructor

This is the first step in rewriting the WellsManager constructor.

In small steps, the rest of the keywords in the constructor will follow, and when finished, all data fetching will take place in the "loop over all wells in the new Schedule object".

The old constructor is unchanged, and there is a test that runs the old and the new, and checks the output well structs for equality.