[Notebook] Update converttonotebook.py

converttonotebook.py was able to recognise a few functions based on a
few hard-coded return types in C++. Now, it searches for patterns such as
   <return type>[*&] [abcd::]abcd(
to find functions, and then it counts () as well as { } to find the
function bodies. It also more reliably deals with things that appear
before function definitions, such as
   template<typename T>

[ROOT-10861] This is a partial fix for ROOT-10861, because functions are
not ripped apart, so the notebook will run.

Further, doxygen-style latex tags such as `\f[` by
notebook-style `$$`, so markdown cells look better.

Make sure converttonotebook recognises indented comments.