math/cadabra2: Update 2.1.6 -> 2.2.0

Port changes:

• Remove the overriding patch of boost::asio: now EINPROGRESS is handled properly, before it was not.
• Remove the overriding patch of websocketpp: added an equivalent patch into cadabra2 source, it is easier this way. Before such function didn't exist in the websocketpp interface.
• Add the patch for pull/82: missing #include / removal of unnecessary prefix calculation.
• Change cmake -> cmake:outsource.
• Switch to python3 because it links to python3 anyway, even though it claims to support python2.
• Add DOS2UNIX_FILES for cmake/version.cmake for one patched file.
• Remove post-patch: added a similar patch under files/
• Remove post-install: stripping is now done by the project.
• Remove post-install-GUI-on: icons are now installed by the project.

Testing:

• Runs fine, shows correct results for examples from the online manual.