Rename add() addColumn() in NTTable

Function for adding columns to table (i.e. to value subfield) was
add(ScalarType). However add(Field) adds extra field to top-level
structure. Rename former to avoid confusion and as more meaningful
name. Update unit tests.

src/nt/nttable.cpp

@@ -20,7 +20,7 @@ static NTFieldPtr ntField = NTField::get();
 
 namespace detail {
 
-NTTableBuilder::shared_pointer NTTableBuilder::add(
+NTTableBuilder::shared_pointer NTTableBuilder::addColumn(
         std::string const & name, epics::pvData::ScalarType scalarType
         )
 {

src/nt/nttable.h

@@ -53,7 +53,7 @@ namespace detail {
          * @param elementType column type, a scalar array.
          * @return this instance of <b>NTTableBuilder</b>.
          */
-        shared_pointer add(std::string const & name, epics::pvData::ScalarType elememtType);
+        shared_pointer addColumn(std::string const & name, epics::pvData::ScalarType elememtType);
 
         /**
          * Add descriptor field to the NTTable.