Use Petrosian ellipse for masks

[burke86]

Currently the masks are roughly defined using a threshold enclosing pixel values normalized to the maximum value of the source. Then, some smoothing is done to the mask. We should investigate using a Petrosian ellipse for a more standardized mask.

The statmorph module has codes to compute it, see '_segmap_gini' code: https://statmorph.readthedocs.io/en/latest/_modules/statmorph/statmorph.html

We may want to implement a similar version of this function.