Add preimage and kernel functionality to BQFClassGroupQuotientMorphism

[TheBlupper]

Extend BQFClassGroupQuotientMorphism to now provide a subgroup for the kernel, and to allow finding a preimage of any class in the codomain. The algorithms are somewhat cobbled together from different sources, SCALLOP gives a nice exposition of the kernel structure, but the concrete implementation of it I had to figure out myself.

I have not seen a description of this preimage algorithm elsewhere, which doesn't require the factorization of the index (or relative conductor, don't know which term I like more).

Some previous discussion on this topic #40143 (comment), where the SCALLOP paper was reccomended by @yyyyx4.

A slightly unrelated issue I noticed, that is not yet addressed here, is that sage.rings.number_field.order::quadratic_order_class_number (which is used by BQFClassGroup.order()) is very slow when the fundamental discriminant is very small but the conductor is large. This should not be the case, as the class number h(f^2*D) can be easily computed from h(D) using these kernel formulas.

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