#290 - Fix slowdown in LinearMap

[schillic]

Closes #290.

[mforets]

Would you mind adding to unit_overapproximate.jl the one disussed in the thread:

function overapproximate_lmap(n)
  B = BallInf(ones(n), 2.)
  π = sparse([1, 2], [1, 2], ones(2), 2, n)
  return Approximations.overapproximate(π*B)
end
o = overapproximate_lmap(50)
@tes o.center == [1., 1] && o.radius == [2., 2]