change Codomain of Located to Point (Codomain a)

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.