Integration of the CCTB into Olex2 has changed

These changes allow the user to have a custom installation/build of cctbx and also use different versions. Although the cctbx management has not changed much under Windows (the user still can use the main menu to install the cctbx plugin, which will be updated by Olex2), there is also a new feature, which allows to override which cctbx installation to use. This is achieved by setting the OLEX2_CCTBX_DIR system environment variable pointing to a valid installation of cctbx. Although that installation will not be managed by Olex2 (such as updating to a new version), we validate the version of the installed cctbx and if the version is too old, a warning will be issued. The use of the system variable is the preferred way to install cctbx on Mac or Linux systems.