reduceunion

 REDUCEUNION Removes redundant elements from a polytope array

 [Pret,kept] = reduceunion(P,Options),

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 DESCRIPTION
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 given a (possibly non-convex) union of polytopes P, removes
 all elements of P which are completely covered by the other regions

 i.e. assume we have 3 polytopes:
      p1 = polytope([0 -1; -1 0; 0 1; 1 0],[1;0;1;1])
      p2 = polytope([0 -1; -1 0; 0 1; 1 0],[1;1;1;0])
      p3 = polytope([0 -1; -1 0; 0 1; 1 0],[1;1;1;1])

 then if Pu=[p1 p2 p3], this function removes polytopes p1 and p2, since they are completely
 covered by a larger polytope (p3 in this case)
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 INPUT
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   P           -   polytope array
   Options     -   will be used when calling regiondiff

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 OUTPUT
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  Pret    -  reduced polytope array
  kept    -  0/1 vector of length(P) which stores a 0 at index i if polytope
             P(i) is redundant and 1 at index i if P(i) is not redundant.