v4.0.0-beta3

This is a beta release of the HOPS4 software. Some minor build system fixes have been introduced. This includes the detection of C++17 support (for the matplot++ dependency), and revision of the python extensions build system. The HOPS python installation is now handled entirely by CMake, and pip is now only called in order to install dependencies (numpy, matplotlib, scipy) if the user toggles on the HOPS_PYPI_MANAGE_DEPS option.