xo-unit: + bpu-class docs

docs/bpu-class.rst

@@ -0,0 +1,32 @@
+.. _bpu-class:
+
+BPU
+===
+
+.. code-block:: cpp
+
+    #include <xo/unit/bpu.hpp>
+
+A :code:`xo::qty::bpu` represents a rational (usually integer) power of a :doc:`basis-unit-class`.
+
+For example:
+
+.. uml::
+    :caption: representation for cubic millimeters
+    :scale: 99%
+    :align: center
+
+    object mm3<<bpu>>
+    mm3 : native_dim = dim::distance
+    mm3 : scalefactor = 1/1000
+    mm3 : power = 3/1
+
+.. uml::
+    :caption: representation for annual (365-day) volatility
+    :scale: 99%
+    :align: center
+
+    object vol<<bpu>>
+    vol : native_dim = dim::time
+    vol : scalefactor = 365*24*3600
+    vol : power = -1/2