gray_indicator() returns incorrect type

[Chuban]

A typo in gray_indicator() (python/adjoint/filters.py) is causing it to return an array. The doc string indicates that this was intended to return a float. I suspect that
return np.mean(4 * x.flatten()) * (1-x.flatten()) * 100
should be
return np.mean(4 * x.flatten() * (1-x.flatten())) * 100

[stevengj]

That sounds right. Care to contribute a PR?