`plotter_helper.average_spectra()` now returns standard errors and standard deviations.

    Create average spectra from a list of spectra.

    Metadata is taken from the first spectrum. Existing y_errors are over-
    written, existing x_errros are not modified.

    Parameters
    ----------
    sp_list : Array of Spectrum objects

    ddof :  int = 1
        Delta Degrees of fredom. The standard deviation is calcualted as
        s = sqrt(1/(N - ddof)*Sum(avg(x)-x_i)), where N is the number of elements

    error_type = : {'st_dev', 'st_err'} = 'st_err'
        The type of error (uncertantiy) returned.
        'st_dev' is the standard deviation of the sample
        'st_err' is the standard  error (st_dev/sqrt(n))
This closes SpectralGroup/spectraplotpy#34
Merge branch 'stderr' of https://github.com/ajasja/spectraplotpy into stderr