ECL peaceman well: replace std::unordered_map by std::map

say 99.95% of all time: premature optimization is the root of all
evil.

in this case it turned out that iterating over a std::unordered_map is
very slow and that this is a quite common operation in
ebos. Fortunately the fix for this problem is very simple: use the
tree-based std::map instead of the hash function based
std::unordered_map.

on SPE9 this yielded a performance improvement of about 10% on my
machine...

applications/ebos/eclpeacemanwell.hh

@@ -35,7 +35,7 @@
 #include <dune/common/version.hh>
 #include <dune/geometry/referenceelements.hh>
 
-#include <unordered_map>
+#include <map>
 
 namespace Opm {
 namespace Properties {
@@ -1414,7 +1414,7 @@ protected:
 
     std::string name_;
 
-    std::unordered_map<int, DofVariables> dofVariables_;
+    std::map<int, DofVariables> dofVariables_;
 
     // the number of times beginIteration*() was called for the current time step
     int iterationIdx_;