Add operator= to dynamic_smoother

Author: djowel

docs/modules/ROOT/pages/reference/misc/dynamic_smoother.adoc

@@ -27,19 +27,20 @@ Notice that, compared to the one-pole lowpass filter, the dynamic smoother respo
 ```c++
    struct dynamic_smoother
    {
-               dynamic_smoother(
-                  frequency base
-                , float sps
-               );
-
-               dynamic_smoother(
-                  frequency base
-                , float sensitivity
-                , float sps
-               );
-
-      float    operator()(float s);
-      void     base_frequency(frequency base, float sps);
+                        dynamic_smoother(
+                           frequency base
+                         , float sps
+                        );
+
+                        dynamic_smoother(
+                           frequency base
+                         , float sensitivity
+                         , float sps
+                        );
+
+      float             operator()(float s);
+      dynamic_smoother& operator=(float y);
+      void              base_frequency(frequency base, float sps);
    };
 ```
 
@@ -67,6 +68,7 @@ Notice that, compared to the one-pole lowpass filter, the dynamic smoother respo
                                       In this case, the sensitivity is set to 0.5.
 | `dynamic_smoother(b)`             | Copy construct a `dynamic_smoother` from `b`.
 | `a = b`                           | Assign `b` to `a`.
+| `a = s`                           | Set the latest result to `s`.
 |===
 
 NOTE: C++ brace initialization may also be used.

q_lib/include/q/fx/lowpass.hpp

@@ -241,6 +241,12 @@ namespace cycfi::q
          return low2;
       }
 
+      dynamic_smoother& operator=(float y)
+      {
+         low1 = low2 = y;
+         return *this;
+      }
+
       void base_frequency(frequency base, float sps)
       {
          wc = as_double(base) / sps;