Indicators as a module (#226)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
  - This PR fixes #216 
- [x] (If applicable) Documentation has been added / updated (for bug
fixes / features).
- [x] (If applicable) Tests have been added.
- [x] CHANGELOG.rst has been updated (with summary of main changes).
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added.

### What kind of change does this PR introduce?

* Makes `indicators` into a module. Moves `pmp.py` into it.

### Does this PR introduce a breaking change?

* The PMP functions are now accessed through `xh.indicators.pmp` rather
than `xh.pmp`.

### Other information:

CHANGELOG.rst

@@ -23,6 +23,7 @@ Breaking changes
 * The `xhydro` testing utilities now require `pytest-xdist` as a development dependency. (:pull:`212`).
 * Many core dependencies have been updated to more modern versions. (:pull:`218`).
 * The `delta_type` argument in ``xhydro.cc.sampled_indicators`` has been renamed to `delta_kind` and is no longer positional. (:pull:`220`).
+* The ``xhydro.pmp`` module has been moved to ``xhydro.indicators.pmp``. (:pull:`226`).
 
 Internal changes
 ^^^^^^^^^^^^^^^^
@@ -31,6 +32,7 @@ Internal changes
 * Several tests reliant on online servers and services have been marked as `online` to prevent them from running in contexts where internet access is limited. (:pull:`212`).
 * Many function docstrings and type hints have been updated for accuracy and precision. (:pull:`212`).
 * The `xHydro` project now has a set of logos for use in documentation, the readme, and other materials. (:pull:`217`).
+* ``xhydro.indicators`` is now a package with submodules for different types of indicators. Previous functions have been moved to a `generic` submodule and are imported at the top level for backwards compatibility. (:pull:`226`).
 
 v0.4.0 (2024-10-04)
 -------------------

docs/notebooks/pmp.ipynb

@@ -27,7 +27,7 @@
     "import numpy as np\n",
     "import xarray as xr\n",
     "\n",
-    "from xhydro import pmp\n",
+    "from xhydro.indicators import pmp\n",
     "from xhydro.testing.helpers import deveraux"
    ]
   },
@@ -433,9 +433,9 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.12.4"
+   "version": "3.12.3"
   }
  },
  "nbformat": 4,
- "nbformat_minor": 2
+ "nbformat_minor": 4
 }

src/xhydro/indicators/__init__.py

@@ -0,0 +1,7 @@
+"""Indicators submodule."""
+
+# Import the submodules
+from . import generic, pmp
+
+# Specific top-level functions
+from .generic import *

tests/test_PMP.py

@@ -4,7 +4,7 @@
 import xarray as xr
 from pint.errors import DimensionalityError
 
-from xhydro import pmp
+from xhydro.indicators import pmp
 
 
 class TestPMP: