EclProblem: add a porosity(elemIdx) method

this is not a generic API but makes emulating legacy stuff which
requires the porosity quite a bit easier in flow_ebos.

ebos/eclproblem.hh

@@ -637,6 +637,16 @@ public:
         return porosity_[globalSpaceIdx];
     }
 
+    /*!
+     * \brief Returns the porosity of an element
+     *
+     * Note that this method is *not* part of the generic eWoms problem API because it
+     * would bake the assumption that only the elements are the degrees of freedom into
+     * the interface.
+     */
+    Scalar porosity(unsigned elementIdx) const
+    { return porosity_[elementIdx]; }
+
     /*!
      * \brief Returns the depth of an degree of freedom [m]
      *