verify shape of all used functions in tests

[skewballfox]

we can avoid making cmvn and other processing functions public until later(since at the moment we only need mfcc), but we should verify that the shape of each output array matches the shape specified in the documentation. This also makes it so that we can safely swap things like

let (mut feature, energy) = mfe(
        signal,
        sampling_frequency,
        frame_length,
        frame_stride,
        num_filters,
        fft_length,
        low_frequency,
        high_frequency,
    );
    ....
    //feature second axis equal to num_filters
    let feature_axis_len = feature.shape()[1];

to just

let feature_axis_len = num_filters;

or just replacing feature_axis_len with num_filters, making the code a bit easier to follow.

[skewballfox]

This has been done for the public functions