docs: "Quantity-like Types" chapter added

Refers to #365
2022-06-14 11:18:29 +02:00
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+.. namespace:: units
+
+Quantity-like Types
+===================
+
+.. raw:: html
+
+    <object data="../_images/quantity_like.svg" type="image/svg+xml" class="align-center" style="max-width: 100%;"></object>
+
+..
+    https://www.planttext.com
+
+    @startuml
+
+    skinparam monochrome true
+    skinparam shadowing false
+    skinparam backgroundColor #fcfcfc
+
+    hide members
+    hide circle
+
+    class quantity [[../framework/quantities.html]]
+    class quantity_point [[../framework/quantity_points.html]]
+    class quantity_kind [[../framework/quantity_kinds.html]]
+    class quantity_point_kind [[../framework/quantity_kinds.html#quantity-point-kinds]]
+
+    Rep <-- quantity : number()
+    quantity <-- quantity_point : relative()
+    quantity <-- quantity_kind : common()
+    quantity_kind <-- quantity_point_kind : relative()
+
+    @enduml
+
+
+[ISO80000]_ defines a :term:`quantity` as a:
+
+    Property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed by
+    means of a number and a reference.
+
+You can use `quantity::number()` member function to get a concrete amount of a unit expressed with a specific
+representation type ``Rep``.
+
+:term:`Kind of quantity <kind of quantity>` is defined as:
+
+    Aspect common to mutually comparable quantities.
+
+We can obtain a `quantity` with a `quantity_kind::common()` member function.
+
+`quantity_point` and `quantity_point_kind` are absolute quantities and quantity kinds relative to some
+specific origin. `quantity` and `quantity_point` types can be obtained from them using a ``relative()`` member
+function (`quantity_point::relative()`, `quantity_point_kind::relative()`).