[TSVB] add _last_series to math metric

[flash1293]

This PR adds params._last_series.<variable name> to the math context which contains an array of all values of the split for the current time step (e.g. if grouped by fieldX which has the values A, B and C and there's a count metric mapped to the variable c, then there would be params._last_series.c which contains a list of 3 counts for A, B and C respectively for the current time step.

This can be used to do a ratio of two series aggs:

• Use group by to split the series
• Fetch the metrics you need
• Do a math agg like params.metric1 / sum(params._last_series.metric2)
• Do a series agg sum

This will give you the ratio of the metric1 and metric2 values summed up per series