Add LanHEP

This adds [LanHEP](http://theory.sinp.msu.ru/~semenov/lanhep.html), which is the program for Feynman rules generation in momentum representation.

It provides a tarball that includes the executable `lhep` and other resources for using in run time. The environment, `LanHEPEnv`, gives the `unpack` function to extract the tarball.