move pyx files to ./engines or ./ext?

[hrahmanin]

'looplib/simlef_onesided.pyx' is not included in setup.py, leading to an error when importing simlef_onesided at the start of this notebook.

todo: just add it to ext_modules

[gfudenberg]

looking at the directory structure, I wonder if all pyx files might be better moved to a folder ./engines ? and then then ./engines/*.pyx could be cythonized? cc @golobor

[gfudenberg]

resolved original issue in 0ea23c8