New "prior" secondary calculation

To ease period over period reporting in some BI tools, an additional secondary calculation could be added.

Example usage:

{{ metrics.metric(
    metric_name='sessions',
    grain='day',
    dimensions=[],
    secondary_calculations=[
        metrics.period_over_period(comparison_strategy="prior", interval=7, alias="sessions_7_days_ago"),
    ],
) }}

This would perform no actual calculation but pull the metric value from the correct interval.

[callum-mcdata]

Sweet! Let me know if you run into any issues and I’m happy to help point you in the right direction. Also a heads up we’re working on a pretty significant overhaul of the package (v0.3.0) in #37 so if you get this over the line before we merge that I’d just convert your integration tests into the new datasets there.

[Aylr]

How exciting!