Documentation improvements. Moved code to filterpy.

Moved the Q_DWPA function to filterpy.

Added help(x) calls for a couple of classes so the docstrings
are displayed in the book. This required me to change the
css style, because it was limiting output to a fairly
short height, and so the help was requiring scrolling.

styles/custom2.css

@@ -98,12 +98,12 @@
     div.output_subarea.output_text.output_pyout {
         overflow-x: auto;
         overflow-y: scroll;
-        max-height: 300px;
+        max-height: 50000px;
     }
     div.output_subarea.output_stream.output_stdout.output_text {
         overflow-x: auto;
         overflow-y: scroll;
-        max-height: 300px;
+        max-height: 50000px;
     }
     code{
       font-size: 70%;