make ParserItems always defaultable

i.e. remove the defaultSet() method and its friends. this is required
to be able to specify defaults in DATA items like grid properties or
saturation tables. e.g.

SGL
10*0.1 10* 10*0.2 /

would not be possible without this. If no meaningful default for an
item is defined, float and double items get NaN, int items get -1 and
string ones get an empty string. The hope is that if these values get
used in the simulation, they will make the result obviously
incorrect. (Whether a data point of an item was defaulted can be
queried using item->defaultApplied(index).)

opm/parser/eclipse/Parser/ParserDoubleItem.cpp

@@ -34,10 +34,14 @@ namespace Opm
             ParserItemSizeEnum sizeType_) :
             ParserItem(itemName, sizeType_)
     {
+        // use NaN as 'default default'. (Keep in mind that in the deck it can be queried
+        // using deckItem->defaultApplied(idx) if an item was defaulted or not...
+        m_default = std::numeric_limits<double>::quiet_NaN();
     }
 
     ParserDoubleItem::ParserDoubleItem(const std::string& itemName) : ParserItem(itemName)
     {
+        m_default = std::numeric_limits<double>::quiet_NaN();
     }
 
 
@@ -54,10 +58,7 @@ namespace Opm
 
 
     double ParserDoubleItem::getDefault() const {
-        if (m_defaultSet) 
-            return m_default;
-        else
-            throw std::invalid_argument("Tried get default from parser item " + name() + " No default has been configured");
+        return m_default;
     }
     
 
@@ -71,6 +72,7 @@ namespace Opm
     ParserDoubleItem::ParserDoubleItem(const Json::JsonObject& jsonConfig) :
             ParserItem(jsonConfig)
     {
+        m_default = std::numeric_limits<double>::quiet_NaN();
         if (jsonConfig.has_item("default")) 
             setDefault( jsonConfig.get_double("default") );
     }