feat: quantity point-like support

docs/CHANGELOG.md

@@ -3,7 +3,7 @@
 - **0.7.0 WIP**
   - (!) refactor: `ScalableNumber` renamed to `QuantityValue`
   - (!) refactor: output stream operators moved to the `units/quantity_io.h` header file
-  - refactor: `quantity_point` (partially) updated to reflect latest changes to `quantity`
+  - refactor: quantity (kind) point updated to reflect latest changes to `quantity`
   - refactor: basic concepts, `quantity` and `quantity_cast` refactored
   - refactor: `abs()` definition refactored to be more explicit about the return type
   - feat: quantity (point) kind support added (thanks [@johelegp](https://github.com/johelegp))

docs/reference/core/customization_points.rst

@@ -8,3 +8,6 @@ Customization Points
 
 .. doxygenstruct:: units::quantity_like_traits
    :members:
+
+.. doxygenstruct:: units::quantity_point_like_traits
+   :members:

docs/use_cases/interoperability.rst

@@ -53,3 +53,14 @@ such an explicit conversion::
 
         using namespace std::chrono_literals;
         quantity q = 1s;  // ERROR
+
+For external quantity point-like types, `quantity_point_like_traits` is also provided.
+It works just like `quantity_like_traits`, except that
+``count(T)`` is replaced with ``relative(T)`` that returns the `QuantityLike` value.
+
+Similar to `quantity` and `quantity_kind`, `quantity_point` and `quantity_kind_point`
+provide a deduction guide from `QuantityPointLike`::
+
+    using namespace std::chrono_literals;
+
+    static_assert(quantity_point{std::chrono::sys_seconds{1s}} + 1_q_s == quantity_point{2s});