Well PI: Use Appropriate Calculator for Each Well

The original code assumed that

    well_container_.size() == numLocalWells()

This assumption does not hold when wells open/shut dynamically in
the context of WECON and/or WTEST.

Switch to indexing into the 'prod_index_calc_' vector using the
well's own linear index instead of manually advancing iterators.

Pointy Hat: [at]bska

opm/simulators/wells/BlackoilWellModel_impl.hpp

@@ -1471,13 +1471,11 @@ namespace Opm {
             return;
         }
 
-        auto piCalc = this->prod_index_calc_.begin();
         for (const auto& wellPtr : this->well_container_) {
             wellPtr->updateProductivityIndex(this->ebosSimulator_,
-                                             *piCalc,
+                                             this->prod_index_calc_[wellPtr->indexOfWell()],
                                              this->well_state_,
                                              deferred_logger);
-            ++piCalc;
         }
     }