Each relevant ConnectorWorkspace now stores an array of pointers to cy_expr
containing the changing symbolic ABCD matrix elements (NULL if not changing).
These are evaluated (with direct pointer access) during the update_parameter_values
routine of each of these workspaces.

This optimisation also allows for a cleaner API for ABCD methods of components,
removing the _sim_cache argument and now just storing the full symbolic and
corresponding numeric matrix in _abcd_matrices dict rather than a bunch of simulation
stuff that needed to be reset / cleared on simulation completion. The method for
updating ABCD matrices outside of simulations remains the same.

Also fixed a bug in cy_expr_init which resulted in inconsistency between changing
parameter target pointers used due to an incorrect method for finding the index.