change: dispersion wl shrinked

the wavelength vector used for dipserion calculation has been shrinked.
The new way to determine dt via wavelength_window made it so that there wasn't 2 extra points to take gradients with.

src/scgenerator/physics/fiber.py

@@ -34,21 +34,14 @@ def lambda_for_envelope_dispersion(
         indices of the original l where the values are valid
         (i.e. without the two extra on each side)
     """
-    su = np.where((l >= wavelength_window[0]) & (l <= wavelength_window[1]))[0]
-    if l[su].min() > 1.01 * wavelength_window[0]:
+    raw_indices = np.where((l >= wavelength_window[0]) & (l <= wavelength_window[1]))[0]
+    if l[raw_indices].min() > 1.01 * wavelength_window[0]:
         raise ValueError(
             f"lower range of {1e9*wavelength_window[0]:.1f}nm is not reached by the grid. "
-            f"Minimum of grid is {1e9*l[su].min():.1f}nm. Try a finer grid"
+            f"Minimum of grid is {1e9*l[raw_indices].min():.1f}nm. Try a finer grid"
         )
-
-    ind_above_cond = su >= len(l) // 2
-    ind_above = su[ind_above_cond]
-    ind_below = su[~ind_above_cond]
-    fu = np.concatenate((ind_below, (ind_below + 2)[-2:], (ind_above - 2)[:2], ind_above))
-    fs = fu[np.argsort(l[fu])[::-1]]
-    l_out = l[fs]
-    ind_out = fs[2:-2]
-    return l_out, ind_out
+    sorted_ind = raw_indices[np.argsort(l[raw_indices])]
+    return l[sorted_ind], sorted_ind[2:-2]
 
 
 def lambda_for_full_field_dispersion(

tests/test_grid.py

@@ -0,0 +1,22 @@
+import numpy as np
+import pytest
+
+import scgenerator as sc
+
+
+def test_scaling():
+    period = 2
+    nt = 512
+    t = sc.tspace(period, nt)
+    w = sc.wspace(t)
+    dt = t[1] - t[0]
+    assert w[1] - w[0] == pytest.approx(2 * np.pi / (period + dt))
+    assert dt == pytest.approx(period / (nt - 1))
+
+
+def test_wl_dispersion():
+    t = sc.tspace(t_num=1 << 15, dt=3.8e-15)
+    w = sc.wspace(t)
+    wl = sc.units.m.inv(w + sc.units.nm(1546))
+    wl_disp, ind_disp = sc.fiber.lambda_for_envelope_dispersion(wl, (950e-9, 4000e-9))
+    assert all(np.diff(wl_disp) > 0)