Generalise 'flat table'; implement DENSITY, PVTW

Several tables are identical both in structure *and* parsing, i.e. the
code required to parse them only differ in output type and possibly
number of items to read.

FlatTable is a simple vector-based template that expands into this exact
parsing routine, based on a simple specification of the record.

opm/parser/eclipse/EclipseState/Tables/TableManager.cpp

@@ -87,6 +87,9 @@ namespace Opm {
         if( deck.hasKeyword( "PVTW" ) )
             this->m_pvtwTable = PvtwTable( deck.getKeyword( "PVTW" ) );
 
+        if( deck.hasKeyword( "DENSITY" ) )
+            this->m_densityTable = DensityTable( deck.getKeyword( "DENSITY" ) );
+
         initVFPProdTables(deck, m_vfpprodTables);
         initVFPInjTables(deck,  m_vfpinjTables);
     }
@@ -637,6 +640,10 @@ namespace Opm {
         return this->m_pvtwTable;
     }
 
+    const DensityTable& TableManager::getDensityTable() const {
+        return this->m_densityTable;
+    }
+
     const TableContainer& TableManager::getMsfnTables() const {
         return getTables("MSFN");
     }