Refactor requirements

[doe300]

Remove direct inclusion of opus and rtaudio or at least move into sub-projects,

1. since they are not compulsory anymore
2. so they don't bloat the repository
3. so they can be updated seperately (and by the user)

Git subprojects would provide a backup if no libraries are available on the installation computer.

• update CMake min version to a newer version supported by Travis CI and Appveyor (3.2)
• Remove all files from git, commit and add subprojects
• Make sure that libraries are still compatible (to be sure, use same librariy-versions as before)
• Need to make sure that OHMComm compiles without the subprojects initialized
• Optionally, modify CMake-script to automatically init subprojects, if no libraries were found. Possible?
• Also update documentation of required libraries, optional or mandatory, what they are used for and where to get them. Add to Readme.
• Add CMake to cryptopp (copy CMakeLists.txt from master similar to opus)
• Fix compilation errors (use FORCE_CUSTOM_LIBRARIES for MSVC, since it doesn't find OpenSSL for some reason)

CMake ExternalProject_Add allows to download and build a git repo without specifying it as submodule. Which way is the better one?

Info here: https://chrisjean.com/git-submodules-adding-using-removing-and-updating/
https://github.com/psi4/psi4/wiki/External-subprojects-using-Git-and-CMake https://cmake.org/cmake/help/v3.0/module/ExternalProject.html