Convexification of a flowpipe

[mforets]

Write a function convexify that receives a flowpipe and an integer k and returns the convexified flowpipe, ie. it groups through ConvexHullArray wrappers the contiguous reach-sets for each chunk of k reach-sets.

[mforets]

there is now Convexify(fp::Flowpipe{N, ReachSet{N, ST}}) where {N, ST}

we could add a variant that also receives an index k >= 1 and convexifies reach-set chunks of size k

[mforets]

on a related note, though L = Convexify(sol[end-k:end]) works, it copies sol[end-k:end] over to a new array. it would be nice if we can avoid the copy. this probably requires a wrapper type ConvexifiedReachSet.