Revert "Ensure that Rep is std::is_arithmetic (#104)" (#111)

This reverts commit db8819c, which
breaks some code I hadn't been aware of in Aurora's repo.  It turns out
that a few people have started using non-arithmetic Rep already, and it
seems to be working out alright.

I also updated the documentation to reflect our "good" status here,
which I elaborated.  I think mp-units still gets "best" here, since they
have a well-defined concept for their Rep.

Fixes #38.

au/quantity.hh

@@ -113,9 +113,6 @@ class Quantity {
     using Unit = UnitT;
     static constexpr auto unit = Unit{};
 
-    static_assert(std::is_arithmetic<Rep>::value,
-                  "Rep must be built-in numeric type for now; see #52");
-
     // IMPLICIT constructor for another Quantity of the same Dimension.
     template <typename OtherUnit,
               typename OtherRep,

docs/alternatives/index.md

@@ -547,10 +547,13 @@ features.
             <a href="https://mpusz.github.io/units/reference/core/concepts.html#_CPPv4I0EN5units14RepresentationE">Representation
             concept</a>
         </td>
-        <td class="fair">
-            Rep can only be <code>is_arithmetic</code> for now, but
-            <a href="https://github.com/aurora-opensource/au/issues/52">plan
-            to upgrade</a>
+        <td class="good">
+            <ul>
+                <li class="check">Mature support for <code>is_arithmetic</code> Rep</li>
+                <li class="check">Experimental support for custom Rep</li>
+                <li class="x">No constraints yet (<a
+                href="https://github.com/aurora-opensource/au/issues/52">#52</a>)</li>
+            </ul>
         </td>
     </tr>
     <tr>