Add signature metrics

[mgdenno]

Under this issue, we want to:

• Split out signatures from our current metrics (i.e., the "metrics" that only take one timeseries, mean, min, max, etc.)
• Create a new class of metrics that first calculates the signature for primary and secondary (based on some initial grouping, event, annual, etc), and then calculates a metric on top of that. A two-step process.
• The returned "metric" could be just the final metric or a dictionary that contains the signatures (A series of signatures) and the signature metric.

There is still something missing in this concept I think. We can currently do this with chained queries I think.

[mgdenno]

This needs some examples identified from the metrics spreadsheet.

[mgdenno]

Not sure if this is mentioned elsewhere, but we should also consider changing the different metric properties to have more explicit names i.e., input_timeseries for signature metrics and primary_input_timeseries and secondary_input_timeseries for comparative metrics because the current pattern is difficult for convey to users. We don't convey that the first value is the primary and the second the secondary, etc.