OGR SQL: don't call CONCAT(a_column ...) or SUBSTR(a_column ...) as a_column; validate that arguments of MAX, MIN, AVG, SUM, COUNT are columns and not any expression since this is not supported; make AVG field definition a OFTReal

git-svn-id: https://svn.osgeo.org/gdal/trunk@26379 f0d54148-0727-0410-94bb-9a71ac55c965