`vertices(::Simplex)` should return `<:AbstractVector`, not `Tuple`

[goretkin]

GeometryTypes.jl/src/convexhulls.jl

Line 21 in 96daf9e

vertices(s::Simplex) = Tuple(s)

I think the rest of the vertices methods return Array{T,1}.

[SimonDanisch]

Fair enough.. Care to make a PR? :)

[CarpeNecopinum]

Think it would be nicer to return an SVector here, since the size of the Simplex is known statically.

It's a bit trickier since we can't yet construct a Simplex from an SVector as we can from a Tuple, but I'm on it. (Down from 30 errors in the tests to 11 already :) )