organize repository

Currently, all scripts and python files are in a single directory: https://gitlab.labs.nic.cz/knot/resolver-benchmarking/tree/master/response_differences/respdiff . It's not clear which files are supposed to be used as modules, which are executable scripts etc.

User experience could also be improved and unified by making the git repo pip-installable.

• separate statistics related functions from dataformat to a submodule
• organize files in respdiff module
• move executable scripts outside respdiff module
• organize (+rename?) the git repository (either create a respdiff-specific one, or move cache_usage_benchmark elsewhere
• update and improve README documentation (create doc/ and separate description for each tool)
• make the git repo pip-installable (create entrypoints for executable scripts)