Sphinx/doxygen C++ API errors

[tartarini]

to replicate:

1. generate the doxygen config
2. create a python vitrtual env and install dependencies : pip install -r requirements.txt
3. run sphinx-build -m html source dest

in addition to a number of warning for python functions signature changes: "line buffering (buffering=1) isn't supported in binary mode"

The following type of errors is generated:

pfire/doc/developer_docs/api.rst:5: WARNING: Error when parsing function declaration. If the function has no return type: Error in declarator or parameters-and-qualifiers Main error: Invalid C++ declaration: Expecting "(" in parameters-and-qualifiers. [error at 40] template<typename mathtype> inline coord< decltype(std::declval< classmathtype >)+std::declval< mathtype >))> operator+ (const coord< mathtype > &a) const ----------------------------------------^ Potential other error: Error in parsing template argument list. If type argument: Invalid C++ declaration: Expected "...>", ">" or "," in template argument list. [error at 81] template<typename mathtype> inline coord< decltype(std::declval< classmathtype >)+std::declval< mathtype >))> operator+ (const coord< mathtype > &a) const ---------------------------------------------------------------------------------^ If non-type argument: Error in postfix expression, expected primary expression or type. If primary expression: Invalid C++ declaration: Expected identifier in nested name, got keyword: decltype [error at 50] template<typename mathtype> inline coord< decltype(std::declval< classmathtype >)+std::declval< mathtype >))> operator+ (const coord< mathtype > &a) const --------------------------------------------------^ If type: Invalid C++ declaration: Expecting '(' or '{' after type in cast expression. [error at 81] template<typename mathtype> inline coord< decltype(std::declval< classmathtype >)+std::declval< mathtype >))> operator+ (const coord< mathtype > &a) const ---------------------------------------------------------------------------------^