Structs
ColourGradient allows you to create custom colour gradients for each PNG created.
Colours required for a PNG file, includes the alpha channel.
A builder struct that will output a spectrogram creator when complete.
This builder will require the height and width of the final spectrogram,
at a minimum. However you can load data from a .wav file, or directly
from a Vec
This contains all the initialised data. This can then produce the spectrogram, and if necessary, save it to the filesystem as a PNG image.