Additionally enhances coreset testing.

Adds a new `recombination.py` module to the solvers subpackage,
providing a generalised `RecombinationSolver`, in addition to the
`CaratheodoryRecombination` and `TreeRecombination` solvers.

A `RecombinationSolverTest` case has been added to `test_solvers.py`,
covering the known degenerate cases that may arise in recombination.

The documentation has been updated where required, but no dedicated
new examples/documentation pages have been created.

Refs: #497

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Performed in response to reviewer comments on #504

Adds references to the original articles for Tchakaloff's theorem and
Caratheodory's convex hull theorem (in French/German respectively), in
addition to more recent, but for our purposes equivalent, references
written in English.

Makes clearer the correspondence between the Caratheodory recombination
algorithm implemented here, and the thesis of Tchernychova. Provides
additional clarity on the steps performed in Tree recombination.

Corrects some simple to fix pyright linting errors.

In attempting to placate the pyright linter a new, erroneous change
was made to the logic in `_resolve_null_basis`. This has been fixed
and the type hints updated appropriately.

Apparent runtime incompatibility with jaxtyping. Appears that the
`Shaped[TypeVar, ...]` syntax used for `_ElementOfOmega is not
supported (even if the TypeVar is bound by Array).