A doctest code block begins with ">>>" and is not indented. Identify
these in doxygen comments and treat them accordingly. Also add check
to padCodeAndVerbatimBlocks for these because Sphinx requires an empty
line before.
Add test case to doxygen_code_blocks.i.
This does somewhat more detailed testing of the code block parsing,
and also exercises the language identification of python doctest
features. For now, it is only tested by python (javadoc translation
may not correctly handle some of the characters that are used here).