Allow asPercent to take multiple series as total

If multiple series as used as total, the length of the total series must
match the length of the seriesList, and they will be matched together to
calculate the percentage of the individual values.

This fixes #207.
Make divideSeries accept multiple divisor series

Remove the current limitation of divisorSeriesList to have only one
series and accept multiple series. In this case the number of divisor
series must match the number of dividend series, and they are matched so
each dividend series get divided by the corresponding divisor.

The old behaviour when only one series is used is retained, so this is
only an extension.