Enable Empty Structural Copies of Deck Items and Deck Keywords

This commit adds a new member function,

    X emptyStructuralCopy() const

to the DeckKeyword and DeckItem classes.  This member function will
form a "structural" copy of "*this", thus preserving aspects such as
the currently active "Dimensions", but removing all data values.
This is in preparation of adding low-pressure table expansion for
the gas PVT tables (i.e., keywords 'PVDG' and 'PVTG').

opm/input/eclipse/Deck/DeckItem.hpp

@@ -47,6 +47,7 @@ namespace Opm {
         DeckItem( const std::string&, double, const std::vector<Dimension>& active_dim, const std::vector<Dimension>& default_dim);
 
         static DeckItem serializationTestObject();
+        DeckItem emptyStructuralCopy() const;
 
         const std::string& name() const;
 
@@ -151,6 +152,7 @@ namespace Opm {
         }
 
         void reserve_additionalRawString(std::size_t);
+
     private:
         mutable std::vector< double > dval;
         std::vector< int > ival;

opm/input/eclipse/Deck/DeckKeyword.hpp

@@ -51,7 +51,7 @@ namespace Opm {
         const std::string& name() const;
         void setFixedSize();
         const KeywordLocation& location() const;
-
+        DeckKeyword emptyStructuralCopy() const;
 
         size_t size() const;
         bool empty() const;

src/opm/input/eclipse/Deck/DeckItem.cpp

@@ -130,6 +130,22 @@ DeckItem DeckItem::serializationTestObject()
     return result;
 }
 
+DeckItem DeckItem::emptyStructuralCopy() const
+{
+    auto ret = *this;
+
+    ret.dval .clear();
+    ret.ival .clear();
+    ret.sval .clear();
+    ret.rsval.clear();
+    ret.uval .clear();
+
+    ret.value_status.clear();
+    ret.raw_data = true;
+
+    return ret;
+}
+
 const std::string& DeckItem::name() const {
     return this->item_name;
 }

src/opm/input/eclipse/Deck/DeckKeyword.cpp

@@ -65,6 +65,15 @@ namespace Opm {
         return result;
     }
 
+    DeckKeyword DeckKeyword::emptyStructuralCopy() const
+    {
+        auto ret = *this;
+
+        ret.m_recordList.clear();
+
+        return ret;
+    }
+
     namespace {
     template <typename T>
     void add_deckvalue( DeckItem deck_item, DeckRecord& deck_record, const ParserItem& parser_item, const std::vector<DeckValue>& input_record, size_t j) {