add mathematical functions module

update docs and clean up code

docs/build/html/_modules/index.html

@@ -35,6 +35,7 @@
   <h1>All modules for which code is available</h1>
 <ul><li><a href="data.html">data</a></li>
 <li><a href="date.html">date</a></li>
+<li><a href="function.html">function</a></li>
 <li><a href="geometry.html">geometry</a></li>
 <li><a href="geometry_plot.html">geometry_plot</a></li>
 <li><a href="numerical/fit.html">numerical.fit</a></li>