xWEL Restart Vectors: Switch to Using Named Indexes

While here, also update pertinent unit tests to use the same vector
identfication scheme.

Suggested by: [at]joakim-hove and [at]atgeirr.

CMakeLists_files.cmake

@@ -493,6 +493,7 @@ if(ENABLE_ECL_OUTPUT)
         opm/output/data/Solution.hpp
         opm/output/data/Wells.hpp
         opm/output/eclipse/VectorItems/intehead.hpp
+        opm/output/eclipse/VectorItems/well.hpp
         opm/output/eclipse/AggregateWellData.hpp
         opm/output/eclipse/CharArrayNullTerm.hpp
         opm/output/eclipse/DoubHEAD.hpp