Support time window in the past in the API

[gregory1g]

**Problem description**
As of now the PopulationDensityData API supports timeframe [now; ) only.
This addresses SORA and potentially some other use cases where future prediction is needed.
However, this excludes use cases related to the historical data

Possible evolution
Suggestion: Extend the API to be able deliver historical data as well.

Additional context
From the API point of view the change promise to be small and backward compatible.
The implementation should be rather simple, because future prediction is already based on historical data.

[gregory1g]

Use cases:

1. social and science studies: for different studies one could need historical data about population in given region, for example to check correlation between studied topics and the population.
2. for drone delivery: an operator can request historical data and use it for its preliminary flight planning which to be validated with the future analyses.
3. Analyses of the past flights and accidents. The regulator can require historical ppl data to analyze it together with the flights and decide if some rules/processes must be adjusted.