Ellipse and circle contour detection produces incorrect results

[StefanoPierini-Codra]

As describer in DataLab-Platform/DataLab#326, ellipse and circle contour detection produces incorrect results.

[StefanoPierini-Codra]

Detected bug described by the issue

In sigima/tools/image/preprocessing.py, the functions fit_ellipse_model and fit_circle_model use scikit-image's EllipseModel and CircleModel.

scikit-image 0.26 introduced a new API with named attributes:

• model.center returns (row, col) = (y, x)
• model.axis_lengths returns (semi_row, semi_col) = (b, a)

The code was reading center[0] as xc and center[1] as yc, but it should be the reverse.

Additional fixes

• wrong formula for ellipsis axis (a minus sign was missing)
• problem with the radius conversion, that was not converted from pixels to physical units

All of them have been fixed.

[ThomasMalletCodra]

Merge in release branch by pull request #22