Default options for time consuming properties

[cruizgo]

Some Graph properties are hard to compute but not always needed, like perfect_matchings. Therefore, we initialize them with a default string which says: This property is not ready, run getPerfectMatchings.

This nooby trick, which is also used in state_catalog and norm, should be replace by this suggestion from Matthias:

class Graph:

    def __init__(self):
        self._state_catalog = None

    @property
    def state_catalog(self):
        if self._state_catalog is None:
            self._compute_state_catalog()
        return self._state_catalog

    def _compute_state_catalog(self):
        # Lengthy calculation here, setting self._state_catalog