demes to ms conversion only supports single-source pulses

[apragsdale]

In some recent changes (#353), we now allow pulses to have multiple sources/proportions (summing to less than 1), to help avoid ambiguity about pulses occurring simultaneously. The demes to ms conversion currently only supports single-source pulses, and raises a ValueError if a pulse has more than one source. This should be fixed to support multiple-source pulses.