EclProblem: Provide Mutable Access to Aquifer Model Object

This commit adds a new public member function,

    EclProblem::mutableAquiferModel()

that returns a read/write reference to the contained EclAquiferModel
object.  The immediate use-case is initializing analytic aquifers
from restart data.

ebos/eclproblem.hh

@@ -1748,6 +1748,9 @@ public:
     EclWellModel& wellModel()
     { return wellModel_; }
 
+    EclAquiferModel& mutableAquiferModel()
+    { return aquiferModel_; }
+
     // temporary solution to facilitate output of initial state from flow
     const InitialFluidState& initialFluidState(unsigned globalDofIdx) const
     { return initialFluidStates_[globalDofIdx]; }