Testing pre-processing functions
This merge request contains changes to the implementation of some of the pre-processing function as well as unit tests to check their performance. The only functions for which unit tests are still missing are filter_isolated_cells and extract_mfcc_features.
I have renamed several of the pre-processing functions. Also, the behavior of the broadband filter is now a little different from before.
I do not fully understand how I am supposed to use filter_isolated_cells. Fabio, could you create a unit test that illustrates its behavior?
Passing all tests and (almost) ready to merge.