Convert axes labels to experiment duration in minutes/seconds

    Notes
    -----
    Matplotlib FuncFormatter function
    https://matplotlib.org/examples/pylab_examples/custom_ticker1.html

def nsamples_to_minsec(x, pos):
    '''Convert axes labels to experiment duration in minutes/seconds

    Notes
    -----
    Matplotlib FuncFormatter function
    https://matplotlib.org/examples/pylab_examples/custom_ticker1.html
    '''
    h, m, s = hourminsec(x/16.0)
    return '{:2.0f}′ {:2.1f}″'.format(m+(h*60), s)