[bluemath_tk/datamining/mda.py] Add option to select first centroid

[tausiaj]

In MDA selection algorithm, it would be useful to select the first centroid in the fit

[AlvaroVal]

When the option for the first centroid is used, the maximum value is still returned. This occurs because, in the validate_data_mda function of decorators.py, the value of first_centroid_seed is not properly referenced in the final return.