Add a Homebrew Formula for BornAgain

A Homebrew formula for building BornAgain is added along with a supporting developer documentation for creation and testing of the formula.

The switch BA_APPLE_HOMEBREW is added to configure the build system to work under Homebrew.

As the Homebrew formula builds the GUI only, BA_PY_PACK switch is re-introduced to avoid building the Python wheel when BA_APPLE_HOMEBREW=ON.

BA Python wheel is changed to a separate target and their build commands are encapsulated in the CMake scripts, instead of explicitly calling a command like bash var/mk_wheel_multilinux.sh.

• Additional modifications
  • In Doc/man/CMakeLists.txt, gzip command is performed after bornagain.1 is made, in order to avoid a build error.
  • Produce the documentation only if BA_DOCS=ON.
  • Unused option LIB_MAN is removed from the documentation.
  • Unneeded postinst.in and prerm.in are removed.