[FIX,DOC] doxygen 1.8.7 fixes

With doxygen 1.8.7 (or maybe earlier) the translated latex code introduces `\+` which is a custom command from  doxygen [1]. I needed to add it to the `refman_overwrite.tex.in` templates otherwise the pdf tutorials could not be build. 

[1] see https://github.com/doxygen/doxygen/commit/c096119b0c0bfc567058538b6225ca26eb191a3a

I also removed the reference to the class search engine as it is no longer available.