Support indexer in schema.yml, add usage example in example.yml (#1561

)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #1522 
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] (If applicable) Documentation has been added / updated (for bug
fixes / features)
- [x] CHANGES.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?

* Adds indicator in example.yml with indexer parameter.
* Adds desription in schema for indexer parameter. 

### Does this PR introduce a breaking change?


### Other information:
Not sure this is all legit, or whether the test suite exercises the
computation of that new indicator.

CHANGES.rst

@@ -15,6 +15,7 @@ New features and enhancements
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 * Added uncertainty partitioning method `lafferty_sriver` from Lafferty and Sriver (2023), which can partition uncertainty related to the downscaling method. (:issue:`1497`, :pull:`1529`).
 * Validate YAML indicators description before trying to build module. (:issue:`1523`, :pull:`1560`).
+* Support ``indexer`` keyword in YAML indicator description. (:issue:`1522`, :pull:`1561`).
 * New ``xclim.core.calendar.stack_periods`` and ``unstack_periods`` for performing ``rolling(time=...).construct(..., stride=...)`` but with non-uniform temporal periods like years or months. They replace ``xclim.sdba.processing.construct_moving_yearly_window`` and ``unpack_moving_yearly_window`` which are deprecated and will be removed in a future release.
 
 Breaking changes

docs/notebooks/example/example.yml

@@ -50,6 +50,12 @@ indicators:
       freq:
         default: YS
     references: ETCCDI
+  spring_fd:
+    compute: frost_days
+    parameters:
+      freq: AS-DEC
+      indexer:
+        month: [ 12, 1, 2 ]
   R95p:
     compute: extreme_precip_accumulation_and_days
     cf_attrs:

pyproject.toml

@@ -149,7 +149,6 @@ values = [
   "release"
 ]
 
-
 [tool.codespell]
 skip = 'xclim/data/*.json,docs/_build,docs/notebooks/xclim_training/*.ipynb,docs/references.bib,__pycache__,*.nc,*.png,*.gz,*.whl'
 ignore-words-list = "absolue,astroid,bloc,bui,callendar,degreee,environnement,hanel,inferrable,lond,nam,nd,ressources,vas"

xclim/data/schema.yml

@@ -19,7 +19,7 @@ indicator:
   missing_options: map(key=str(), required=False)
   notes: str(required=False)
   cf_attrs: any(list(include('cf_attrs')), include('cf_attrs'), required=False)
-  parameters: map(str(), num(), bool(), null(), include('parameter'), key=str(), required=False)
+  parameters: map(str(), num(), bool(), null(), include('parameter'), include('indexer'), key=str(), required=False)
   realm: str(required=False)
   references: str(required=False)
   title: str(required=False)
@@ -29,10 +29,18 @@ cf_attrs: map(str(), key=str(), required=False)
 
 parameter:
   description: str(required=False)
-  default: any(str(), num(), bool(), null(), required=False)
+  default: any(str(), num(), bool(), null(), include('indexer'), required=False)
   choices: list(str(), required=False)
   units: str(required=False)
 
+indexer:
+  drop: bool(required=False)
+  month: any(int(), list(int()), required=False)
+  season: any(str(), list(str()), required=False)
+  doy_bounds: list(int(), required=False, maxItems=2)
+  date_bounds: list(str(), required=False, maxItems=2)
+  include_bounds: any(bool(), list(bool(), bool()), required=False, maxItems=2)
+
 variable:
   canonical_units: str(required=True)
   cell_methods: str(required=False)