Write docs for unit API module

src/ifcopenshell-python/ifcopenshell/api/unit/add_context_dependent_unit.py

@@ -18,11 +18,40 @@
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, unit_type="USERDEFINED", name="THINGAMAJIG", dimensions=None):
+        """Add a new arbitrary unit that can only be interpreted in a project specific context
+
+        Occasionally the construction industry uses arbitrary units to quantify
+        objects, like "pairs" of door hardware, "palettes" or "boxes" of fixings
+        or equipment.
+
+        :param unit_type: Typically should be left as USERDEFINED, unless for
+            some bizarre reason you are redefining something you could use a
+            sensible normal unit for. In that case, firstly stop whatever you're
+            doing and have a hard think about your life, and then if life really
+            is going that badly for you, check out the IFC docs for IfcUnitEnum.
+        :type unit_type: str
+        :param name: Give your unit a name. X what? X bananas?
+        :type name: str
+        :param dimensions: Units typically measure one of 7 fundamental physical
+            dimensions: length, mass, time, electric current, temperature,
+            substance amount, or luminous intensity. These are represented as a
+            list of 7 integers, representing the exponents of each one of these
+            dimensions. For example, a length unit is (1, 0, 0, 0, 0, 0, 0),
+            where as an area unit is (2, 0, 0, 0, 0, 0, 0). A unit of meters per
+            second is (1, 0, -1, 0, 0, 0, 0). For context dependent units, it is
+            recommended to leave this as the default of (0, 0, 0, 0, 0, 0, 0).
+        :type dimensions: list[int]
+        :return: The new IfcContextDependentUnit
+        :rtype: ifcopenshell.entity_instance.entity_instance
+
+        Example::
+
+            # Boxes of things
+            ifcopenshell.api.run("unit.add_context_dependent_unit", model, name="BOXES")
+        """
         self.file = file
-        self.settings = {"unit_type": "USERDEFINED", "name": "THINGAMAJIG", "dimensions": (0, 0, 0, 0, 0, 0, 0)}
-        for key, value in settings.items():
-            self.settings[key] = value
+        self.settings = {"unit_type": unit_type, "name": name, "dimensions": dimensions or (0, 0, 0, 0, 0, 0, 0)}
 
     def execute(self):
         return self.file.create_entity(