Chamberlin: change formula to latex graphic

Replace the text formula with a png graphic generated from some
LaTeX source.
Formulas in HTML apparently never made it in the mainstream,
and the recommendation is to use LaTex to generate images. See:
https://www.w3.org/MarkUp/HTMLPlus/htmlplus_45.html

To convert the .tex to a png, try:
  pdflatex formulas.tex
  convert -density 300 formulas.pdf -quality 95 formulas.png

Signed-off-by: Graham Whaley <graham.whaley@gmail.com>

gui/formulas.tex

+% Generate plain formulas to be turned into png files via a pdf. Code hints taken
+% from:
+%
+% https://tex.stackexchange.com/questions/287481/using-latex-to-generate-png-images-of-an-equation/287501
+%
+\documentclass[multi={mymath},border=1pt]{standalone}
+% \usepackage{amsmath}
+\newenvironment{mymath}{$\displaystyle}{$}
+
+\begin{document}
+
+\begin{mymath}
+  F = Fcenter * 2^{(signal * octaves)}
+\end{mymath}
+
+\end{document}

gui/index.html

 		frequency is:
 	</p>
 	<p>
-		F = Fcenter * 2^<sup>(signal * octaves)</sup>
-		<br><small>If anyone knows how to do HTML equations, please
-		help me improve this.....</small>
+		<img src="img/filter_formula.png">
 	</p>
 	<p>When operating with signal control of corner frequency, this
 		object uses approximately 4% of the CPU time on Teensy 3.1.