matroids: Add certificate argument to is_valid

[gmou3]

When certificate is set to True, the method is_valid returns a tuple of a boolean and a dictionary.
The dictionary provides some more information in case the matroid is not valid.

Examples:

sage: M = matroids.catalog.Fano()
sage: M.is_valid()
True
sage: M.is_valid(certificate=True)
(True, {})

sage: C = [[1, 2, 3], [3, 4, 5]]
sage: M = Matroid(circuits=C)
sage: M.is_valid()
False
sage: M.is_valid(certificate=True)
(False,
 {'circuit 1': frozenset({3, 4, 5}),
  'circuit 2': frozenset({1, 2, 3}),
  'element': 3,
  'error': 'elimination axiom failed'})

To be done constistently with #38024.

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[mkoeppe]

That's a nice addition, thanks.

Could it be a good idea to have an iterator that provides the errors?

[gmou3]

From my experience, one usually needs only one failure in order to unravel the invalidity and fix it.

Although it is possible that having all errors provides some utility, I think that would be marginal; one not worth the code complication.

[tscrim]

LGTM.

[gmou3]

Thanks!