Register percent and permille units (#359)

* Register percent and permille units

Signed-off-by: F.N. Claessen <felix@seita.nl>

* Changelog entry

Signed-off-by: F.N. Claessen <felix@seita.nl>

* Add test case

Signed-off-by: F.N. Claessen <felix@seita.nl>

documentation/changelog.rst

@@ -15,6 +15,7 @@ New features
 * Add CLI commands ``flexmeasures add sensor``, ``flexmeasures add asset-type``, ``flexmeasures add beliefs`` (which were experimental features before). [see `PR #337 <http://www.github.com/FlexMeasures/flexmeasures/pull/337>`_]
 * Add CLI commands for showing data [see `PR #339 <http://www.github.com/FlexMeasures/flexmeasures/pull/339>`_]
 * Add CLI command for attaching annotations to assets: ``flexmeasures add holidays`` adds public holidays [see `PR #343 <http://www.github.com/FlexMeasures/flexmeasures/pull/343>`_]
+* Support for percent (%) and permille (‰) sensor units [see `PR #359 <http://www.github.com/FlexMeasures/flexmeasures/pull/359>`_]
 
 Bugfixes
 -----------

flexmeasures/utils/tests/test_unit_utils.py

@@ -20,6 +20,7 @@
 @pytest.mark.parametrize(
     "from_unit, to_unit, expected_multiplier, expected_values",
     [
+        ("%", "‰", 10, None),
         ("m/s", "km/h", 3.6, None),
         ("m³/h", "l/h", 1000, None),
         ("m³", "m³/h", 4, None),

flexmeasures/utils/unit_utils.py

@@ -31,8 +31,16 @@
 )
 
 # Set up UnitRegistry with abbreviated scientific format
-ur = pint.UnitRegistry(full_template)
+ur = pint.UnitRegistry(
+    full_template,
+    preprocessors=[
+        lambda s: s.replace("%", " percent "),
+        lambda s: s.replace("‰", " permille "),
+    ],
+)
 ur.default_format = "~P"  # short pretty
+ur.define("percent = 1 / 100 = %")
+ur.define("permille = 1 / 1000 = ‰")
 
 
 PREFERRED_UNITS = [