Better linear_map of AbstractZonotope for 1D output

[schillic]

If the output dimension is 1, we may still want to return a Zonotope for type stability. But the result can have just one generator.

julia> @time linear_map(M, Z)  # master
  0.000015 seconds (3 allocations: 176 bytes)
Zonotope{Float64, Vector{Float64}, Matrix{Float64}}([0.6565135067379049], [0.06137160508809101 -1.0290195455026827 -0.29127884051790404])

julia> @time linear_map(M, Z)  # this branch
  0.000011 seconds (3 allocations: 160 bytes)
Zonotope{Float64, Vector{Float64}, Matrix{Float64}}([0.6565135067379049], [-1.2589267809324955;;])