Review how index computation is performed

- Add (.yaml) config files where all of the key parameters are defined. No constants should be defined in the Python!
- Could specify better args at higher levels, and no defaults at low-levels? perhaps better to modularise and avoid nesting calls
- Maybe have a default spectogram code in the .yaml and then allow the individual indices to use different ones.
- Factory method to initialise functions with parameters set by the configuration file? Create a static object.