Doxygen 1.8.13

Notice that there are a few errors in some formulas.
To see all the warnings, look in the raw log from travis ci.
I also opened an issue right now (#213).

Anyway, I checked and the same errors were in the previous manual
(with doxygen 1.8.12). I thus upgrade to latest doxygen.

[makedoc]

.travis.yml

@@ -98,7 +98,7 @@ install:
 # then replace doxygen with the desided version
 # I use 1.8.10 instead of 1.8.11 since it looks like 1.8.11 have troubles with
 # non case sensitive files (it writes capitalized filenames)
-  - if test "$MAKEDOC" == yes ; then ./.travis/install.doxygen Release_1_8_12 ; fi
+  - if test "$MAKEDOC" == yes ; then ./.travis/install.doxygen Release_1_8_13 ; fi
   - if test "$VALGRIND" == yes ; then sudo apt-get install -y valgrind           ; fi
   - if test "$LAPACK" == yes ; then sudo apt-get -y install libatlas-base-dev      ; fi
 # moreover, we hardcode path to dynamic library, required for xdrfile to link properly