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>

Note in the GUI help text that the ToneDetect uses the
Goertzel algorithm.

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

https://forum.pjrc.com/threads/52679-Teensyduino-1-42-Released?p=181195&viewfull=1#post181195

https://forum.pjrc.com/threads/45246-Issues-with-tonesweep-in-audio-library

https://forum.pjrc.com/threads/27806-Clicking-envelope-with-Audio-Library

fixes #237