Public API for weights

Sometimes I want to interpolate between objects, that do not implement `+` or `*`, but for which convex combinations still make sense. A typical example is that I have a probability distribution attached to each grid point. Another example would be a rotation attached to each grid point (these are not really a convex set, but there are still some useful notions of weighted average for them).

It would be nice to reuse the weight computation of this package in such cases. I am thinking about something like this:
```julia
A = # some array of probability distributions
itp = interpolate(knots, A, Gridded(Linear())) # this is a bit strange, we can't do itp(pt...), maybe deserves its own object.
wts = WeightedIndex(itp, pt)

# convex_combination is a user define function that Interpolations.jl does not know about
convex_combination(wts)
```
Any thoughts?