`get_isosurface_1d` does not work as expected

[OnnoEbbens]

I try to calculate isosurfaces but I don't get the results I expect when the isovalue is crossed multiple times.

>>> import numpy as np
>>> import nlmod
>>> nlmod.layers.get_isosurface_1d(np.array([10, 20, 30, 30, 10]),
                                   np.array([-1, -2, -3, -4, -5]),
                                   25)
np.float64(-5.0)

It works fine when the isovalue is crossed one time

>>> nlmod.layers.get_isosurface_1d(np.array([10, 20, 30, 30, 40]),
                                   np.array([-1, -2, -3, -4, -5]),
                                   25)
np.float64(-2.5)

The docstring states:

This function interpolates linearly along z, if da crosses the given interpolation
value at multiple depths, the first elevation is returned.
but this doesn't seem to be the case

[dbrakenhoff]

Good catch! I think the docstring was written for an earlier version of the function using scipy's interp1d, which sorts the data prior to performing the interpolation. This is also not a good solution however, so I will write a new version.