Mistake in viz.compute_cdf()?

Hi!

I think there is a small error in the way the cdf is computed. I think it should be like this.
def compute_cdf(pdf):
"""Compute the cumulative distribution function (cdf) given a pdf."""
values = np.array([np.trapz(pdf.iloc[:x**+1**], pdf.index[:x**+1**]) for x in range(pdf.size)])
cdf = pd.Series(values, index=pdf.index.values)
return cdf

Before the first two values were 0, because when x=0 there are no points in pdf.iloc[:0] and the integral over one point pdf.iloc[:1] is 0 and the last points wasn't evaluated. This should be changed in future versions of pyam.