Calculate alpha values from the coverage/2.5.

def alpha_from_cov(plotman, alpha_cov):
    '''Calculate alpha values from the coverage/2.5.
    '''
    abscov = np.abs(load_cov('inv/coverage.mag'))
    if alpha_cov:
        normcov = np.divide(abscov, 2.5)
        normcov[np.where(normcov > 1)] = 1
        mask = np.subtract(1, normcov)
        alpha = plotman.parman.add_data(mask)
    else:
        alpha = plotman.parman.add_data(np.ones(len(abscov)))
    return alpha, plotman