small improvements to StandardWellsDense
the most significant change is that only a "PhaseUsage" object must be passed to its constructor instead of a full "fluid object". also, the pointers to the vector of "active" phases has been converted into a full object. (this helps to avoid potential use-after-free errors.)
jenkins run norne_ebos please
the CI systems are happy, yay! @totto82: any objections to merging this?
Looks simpler, better. Feel free to merge.