Return a dictionary of popular features used for EMG time series classification.

def emg_features(threshold=0):
    '''Return a dictionary of popular features used for EMG time series classification.'''
    return {
        'mean_abs_value': mean_abs,
        'zero_crossings': zero_crossing(threshold),
        'slope_sign_changes': slope_sign_changes(threshold),
        'waveform_length': waveform_length,
        'integrated_emg': abs_sum,
        'emg_var': emg_var,
        'simple square integral': abs_energy,
        'root_mean_square': root_mean_square,
        'willison_amplitude': willison_amplitude(threshold),
    }