molssi-seamm/molsystem: Add handling of strain and improved handling of properties

Added methods for straining the unit cell, and also straining a configuration, correctly handling the coordinates for an affine transformation. In the future will add e.g. affine transformation of the centers of molecules, which is useful for molecular fluids.

Added the system for properties, in addition to the configuration. This allows system properties that are not associated with a particular configuration, which is often appropriate for experimental results. It also makes it much easier to search for systems where any configuration has a particular property.