Added audio FFT method

* Added @audioMat class with plot and process methods and an audio2FFT
package method
* Sped up chunking using a different method
* Added setSecondsPerChunk “stream” method for increased stability
* Added audio2chunks, openAudio, readAudio to “stream” to cope with
audio data
Added a superclass plotting method

* The superclass plot method takes care of creating a figure and
storing its handle
# only apparent glitch: abstract cannot remain abstract [i.e. required
in subclass]