Do not use the well's comp_frac member, only rely on the control's distr
member for initialization. This forced a change to WellsManager's
initialization of the distr member.
This is a minor bugfix to account for the presence of shut wells
(characterised by "ctrls[w]->current < 0"). The existing code would
lead to indexing outside the "ctrls" array in the context of a shut
well.
Bhp is now initialized to bhp target for bhp-controlled wells.
Mobilities and pvt properties are now calculated from
well perforation pressure and injection specifications for
injectors, producers still use cell properties as before.
