Add revised peak_local_max to skimage2 (scikit-image#8039)

Adds the function `peak_local_max` to the `skimage2` namespace. Compared
to the old version in `skimage` it has a few differences:

* Parameter `p_norm` defaults to 2 (Euclidean distance),
  was `numpy.inf` (Chebyshev distance)
* Parameter `exclude_border` defaults to 1, was `True`
* Parameter `exclude_border` no longer accepts `False` and `True`,
  pass 0 instead of `False`, or `min_distance` instead of `True`
* Parameters after `image` are keyword-only

Deprecates passing `np.inf` to `num_peaks` and 
`num_peaks_per_label`.

In `skimage.feature.peak_local_max`, support passing floats to the
parameter `min_distance`.

In `skimage.feature.peak_local_max`, use `None` as the new default for the
parameters `num_peaks` and `num_peaks_per_label` – the default behavior is
the same as before but passing `numpy.inf` is deprecated.

Ensure `skimage.feature.peak_local_max` returns results with the correct shape
when no peaks where found and `labels` where given.
---------

Co-authored-by: Stefan van der Walt <stefan@mentat.za.net>

Author: lagru

doc/source/user_guide/skimage2_migration.md

@@ -65,6 +65,57 @@ ski2.data.binary_blobs(
 with `length`, `n_dim`, and `blob_size_fraction` containing values used with the old signature.
 Other parameters -- including `boundary_mode` if you already set it explicitly -- can be left unchanged.
 
+### `skimage.feature.peak_local_max`
+
+This function is replaced by `skimage2.feature.peak_local_max` with new behavior:
+
+- Parameter `p_norm` defaults to 2 (Euclidean distance), was `numpy.inf` (Chebyshev distance)
+- Parameter `exclude_border` defaults to 1, was `True`
+- Parameter `exclude_border` no longer accepts `False` and `True`, pass 0 instead of `False`, or `min_distance` instead of `True`
+- Parameters after `image` are keyword-only
+
+To keep the old behavior when switching to `skimage2`, update your call according to the following cases:
+
+:::{list-table}
+:header-rows: 1
+
+- - In `skimage`
+  - In `skimage2`
+
+- - `exclude_border` not passed (default)
+  - Assign it the same value as `min_distance` which may be its default value `1`.
+
+- - `exclude_border=True`
+  - Same as above in the default case.
+
+- - `exclude_border=False`
+  - Use `min_distance=0`.
+
+- - `exclude_border=<int>`
+  - No change necessary.
+
+- - `p_norm` not passed (default)
+  - Pass the previous default explicitly with `p_norm=numpy.inf`.
+
+- - `p_norm=<float>`
+  - No change necessary.
+
+:::
+
+Other keyword parameters can be left unchanged.
+
+Examples:
+
+```python
+ski.morphology.peak_local_max(image)
+ski2.morphology.peak_local_max(image, exclude_border=1, p_norm=np.inf)
+
+ski.morphology.peak_local_max(image, min_distance=10)
+ski2.morphology.peak_local_max(
+    image, min_distance=10, exclude_border=10, p_norm=np.inf
+)
+```
+
 ## Deprecations prior to skimage2
 
 We have already introduced a number of changes and deprecations to our API.

pyproject.toml

@@ -195,7 +195,8 @@ addopts = [
     "--strict-config",
     "--strict-markers",
     "--maxfail=5",
-    # Necessary for doctest collection with editable installations
+    # Necessary for doctest collection with editable installations and
+    # non-unique test module names (skimage/skimage2 duplicates those)
     "--import-mode=importlib",
     # See comment on `testpaths`
     "--pyargs",

src/skimage/feature/corner.py

@@ -1201,6 +1201,8 @@ def corner_peaks(
     """
     if np.isinf(num_peaks):
         num_peaks = None
+    if np.isinf(num_peaks_per_label):
+        num_peaks_per_label = None
 
     # Get the coordinates of the detected peaks
     coords = peak_local_max(
@@ -1209,7 +1211,7 @@ def corner_peaks(
         threshold_abs=threshold_abs,
         threshold_rel=threshold_rel,
         exclude_border=exclude_border,
-        num_peaks=np.inf,
+        num_peaks=None,  # Limiting to `num_peaks` is done in this function
         footprint=footprint,
         labels=labels,
         num_peaks_per_label=num_peaks_per_label,