#1051: restructure the root directory

This change consists of 4 parts:
- move around directories, for instance move all the C++ into casadi/casadi, and rename cmake_modules -> cmake
- update CMakeLists to reflect this
- update tons of headers for example "symbolic/blah" to "casadi/symbolic/blah", including swig stuff
- update the readme to explain the new directory structure

there were no changes to the code

I tried to update all the headers, but I can't build some things like cplex,
so this commit works for me but this probably introduces a few bugs where "symbolic/blah" needs
to be changed to "casadi/symbolic/blah".