FaultSlip: interpolation error if 0 < l_difference < 1

[VvanderHeiden]

When the slip patch length is approaching the theoretical nucleation length but still above zero, we cannot interpolate as there is not 'zero crossing'.

ERROR:
Error using interp1 (line 188)
Interpolation requires at least two sample points for each grid dimension.

Error in FaultSlip/detect_nucleation (line 174)
nucleation_per_slip_zone(j) = interp1(l_difference(unique_index), x(unique_index), 0);

Error in panther (line 79)
parfor (i = 1 : n_members, matlab_workers)

Error in MultiFaultCalculator/run (line 40)
parfor i = 1 : n

SOURCE:
https://github.com/TNO/PANTHER/blob/f19dad44702c84555740b8d4290f35639ba24c15/src/calculator/FaultSlip.m#L163C21-L163C72