This function needs a better name!

…ion name)

Update tests (and add a new one): so progress, but not good enough.
The mathematical tests also need fixing first.

Co-authored-by: Thomas R. Murrills <68410468+thorimur@users.noreply.github.com>

These are not exhaustive, but probably a good start.

…ct space,

return that space and its base field

A future commit will use this to warn on the product of two models with corners
on normed spaces (as there are two potential models on it, which are not
definitionally equal).

The inner loop handles finding a model on a non-product, and the
outer loop will just be responsible for products, disjoint unions
and open subsets of a manifold.

… open subsets

And update diagnostics for slight error message tweaks (for the better).

This makes the code clearer (and also makes it easier to change which
information we store): reverting to e.g. just a Boolean is easier now.