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.

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)