change Codomain of Located to Point (Codomain a)

[byorgey]

Switch from

Codomain (Located a) = Located (Codomain a)

to

Codomain (Located a) = Point (Codomain a)

When calling atParam on e.g. a Located (Trail R2), I expect to get a
P2, not a Located R2 (though of course the latter can be converted to
the former).

This also means that instead of requiring constraints of the form

V (Codomain a) ~ V a

we instead have

Codomain a ~ V a

which makes more sense: we can only add a location to
translation-invariant things, and translation-invariant parametric
things are precisely those things whose codomain is a vector space.

@fryguybob , does this make sense to you? I think you're the one who has worked with Located the most.

[fryguybob]

This makes sense to me. I've updated the expand branch and this allows me to use atEnd and atStart directly. The notion that only translation-invariant things should be located leads to a specialized mapLoc when you are mapping to something located: mapLoc :: V a ~ V b => (a -> b) -> Located a -> Located b with b ~ Located c gives mapLoc :: V a ~ V c => (a -> Located c) -> Located a -> Located (Located c). In Diagrams.TwoD.Offset in the expand branch I have bindLoc but I don't know if the type for it is too general. I think what we want is bindLoc :: V a ~ V c => (a -> Located c) -> Located a -> Located c. Anyway, I'll merge this now.

[byorgey]

Yes, something like bindLoc :: V a ~ V c => (a -> Located c) -> Located a -> Located c makes sense to me.