(#437) Using MD or TVD depth values for well log extraction curves

Also renamed some methods for clarity, and fixed a bug in
RimWellLogPlot::updateViewerWidget(). Using this method instead of
loadDataAndUpdate() when toggling visibility of the plot.

ApplicationCode/ProjectDataModel/RimWellLogPlot.h

@@ -56,6 +56,7 @@ public:
     size_t  traceCount() { return traces.size();}
 
     void loadDataAndUpdate();
+    void updateTraces();
 
     RiuWellLogPlot* viewer();
 
@@ -71,6 +72,8 @@ public:
     void updateAxisRanges();
     void setVisibleDepthRangeFromContents();
 
+    DepthTypeEnum depthType() const;
+
     virtual caf::PdmFieldHandle* userDescriptionField()  { return &m_userName; }
 
 protected: