Refactor + small improvements in order reduction functions

[mforets]

julia> Z = Zonotope(rand(4), rand(4, 8) .- 0.5);

julia> Zst = Zonotope(SVector{4}(Z.center), SMatrix{4, 8}(Z.generators));

julia> @btime ReachabilityAnalysis._reduce_order($Z, 1); # master
  525.942 ns (19 allocations: 1.50 KiB)

julia> @btime ReachabilityAnalysis._reduce_order($Zst, 1);  # master
  418.739 ns (11 allocations: 1.20 KiB)

julia> @btime ReachabilityAnalysis._reduce_order($Z, 1);   # new
  321.116 ns (8 allocations: 624 bytes)

julia> @btime ReachabilityAnalysis._reduce_order($Zst, 1); # new
  273.169 ns (7 allocations: 560 bytes)