Added plotter_helper.baseline_correct

    Substracts the baseline in place.
    Currently only a polynomial baseline is supported.

    Parameters
    ----------
    spectrum :
        spectrum-like object
    left_num = 100 :
        Number of point on the left side to take as baseline for fitting.
    right_num = 100 :
        Number of point on the right side to take as baseline for fitting.
    deg  = 3 :
        The degree of the polynomial.