Add horizontal_tilt_angle function and tests

[ErosKerouak]

This pull request adds the horizontal_tilt_angle function to harmonica._transformations.

The horizontal tilt angle is a enhancement filter used in gravity and magnetic interpretation. It computes the arctangent of the horizontal gradient magnitude over the absolute value of the upward derivative.

Summary of changes:

• Implemented horizontal_tilt_angle function with full docstring and LaTeX formula
• Added unit tests based on a synthetic model
• Input validation for grids with invalid dimensions and NaN values

Relevant issues/PRs:
Closes #559
Related to #560, #561

[gaoflow]

The Actions logs for the failing code-style jobs have expired, but I was able to reproduce the style issues locally on the PR checkout using this branch's older Makefile layout (harmonica doc, not current src/harmonica test doc).

The failures are concentrated around the new horizontal_tilt_angle code and tests: import ordering, blank-line spacing, docstring line length, one trailing whitespace, and Black formatting.

A minimal patch is:

diff --git a/harmonica/_transformations.py b/harmonica/_transformations.py
@@
 """
 Apply transformations to regular grids of potential fields
 """
+
 import numpy as np
@@
     Compute the amplitude of the horizontal gradient of a regular gridded
     potential field `M`. This is a measure of the rate of change in the x and y
-    (horizontal) directions. . The horizontal derivatives are calculated though 
+    (horizontal) directions. The horizontal derivatives are calculated though
     finite-differences.
@@
-        order: *northing*, *easting*. The coordinates must be defined in the same units.
+        order: *northing*, *easting*. The coordinates must be defined in the
+        same units.
@@
-        A :class:`xarray.DataArray` containing the total horizontal derivative of
-        the input ``grid``.
+        A :class:`xarray.DataArray` containing the total horizontal derivative
+        of the input ``grid``.
@@
     horizontal_gradient = (
         derivative_easting(grid, order=1),
-        derivative_northing(grid, order=1)
+        derivative_northing(grid, order=1),
     )
@@
     horiz_deriv = np.sqrt(gradient[0] ** 2 + gradient[1] ** 2)
     return np.arctan2(horiz_deriv, total_gradient)
 
+
 def horizontal_tilt_angle(grid):
@@
-    horizontal gradient magnitude and the absolute value of the upward derivative.
+    horizontal gradient magnitude and the absolute value of the upward
+    derivative.
@@
-
-
 def _get_dataarray_coordinate(grid, dimension_index):
diff --git a/harmonica/tests/test_transformations.py b/harmonica/tests/test_transformations.py
@@
 """
 Test functions for regular grid transformations
 """
+
 from pathlib import Path
@@
     derivative_upward,
     gaussian_highpass,
     gaussian_lowpass,
+    horizontal_tilt_angle,
     reduction_to_pole,
-    tilt_angle,
     theta_map,
-    horizontal_tilt_angle,
+    tilt_angle,
@@
-    def test_against_synthetic(
-        self, sample_potential, sample_g_n, sample_g_e
-    ):
+    def test_against_synthetic(self, sample_potential, sample_g_n, sample_g_e):
@@
         with pytest.raises(ValueError, match="Found nan"):
             theta_map(sample_potential)
 
+
 class TestHorizontalTiltAngle:
@@
-        Check if horizontal_tilt_angle raises error on grid with single dimension
+        Check if horizontal_tilt_angle raises error on grid with single
+        dimension
@@
-        Check if horizontal_tilt_angle raises error on grid with three dimensions
+        Check if horizontal_tilt_angle raises error on grid with three
+        dimensions
@@
-
 class Testfilter:

Verified locally on the changed files:

isort --check harmonica/_transformations.py harmonica/tests/test_transformations.py
flake8 harmonica/_transformations.py harmonica/tests/test_transformations.py
black --check harmonica/_transformations.py harmonica/tests/test_transformations.py
git diff --check

I couldn't run the new tests in this local environment because verde is not installed, and a full install of the old style requirements hit local disk limits. So this is scoped to the style failures only.