implement extract_contour.py for IPOBO build

[tomsail]

For very big meshes, the selafin xarray export might take considerable time because of the IPOBO calculation.

Boris Basic has provided a script that enhanced significantly the processing time for extracting contour information.

https://gitlab.pam-retd.fr/otm/telemac-mascaret/-/blob/main/scripts/python3/pretel/extract_contour.py

the current function

xarray-selafin/xarray_selafin/Serafin.py

Line 677 in 6c99f28

def iter_on_boundaries(self):

needs to be compared to extract_contour.py performances