Add Wells::comp_frac for producers

This adds code to set the comp_frac data member of producing wells to represent the preferred phase of the well. Previously, this was set to all zeros.

This is to avoid a few somewhat tricky problems downstream (in opm-autodiff's fully implicit black-oil solver).

One should note that previously, the policy was that comp_frac was only valid for injection wells. This should no longer be considered true. The documentation of Wells::comp_frac has been changed accordingly.

Note: this PR requires OPM/opm-parser#172.