Add get_x_l_ratio() method to BaseSurface for PFDHA applications

[vup1120]

This PR adds support for computing normalized along-strike position (x/L) for Probabilistic Fault Displacement Hazard Assessment (PFDHA) applications.

It includes:

1. _get_surface_trace():
   Private helper method that extracts the surface trace (top edge) as a 2D array of geographic coordinates with NaN
   handling for KiteSurface compatibility.
2. get_x_l_ratio(mesh): It computes the normalized along-strike position (x/L) for each point in the mesh relative to the surface trace
3. Testing suite added to base_surface_test.py

Notes:

1. Point-to-polyline projection algorithm finds the nearest trace segment
   for each site using vectorized distance calculations
2. Returns tuple (x_over_L, L_km) where x_over_L is a numpy array with
   values in [0, 1] and L_km is the total trace length in kilometers
3. All x/L values are clamped to [0, 1] before return

[micheles]

How can this be negative?

[vup1120]

I removed this and only keep the if L_proj == 0.0 check ( at #284). It was if L_proj <= 0.0 before, but it actually won't be negative, so I modified it now to "== 0.0"

[mmpagani]

Thanks @vup1120. The PR looks good to me but you need to fix the conflict in the changelog

[mmpagani]

I would call this _get_tor(self)

[vup1120]

All Fixed 🙂