Improving data samples export

[fedpet]

I have 2 propositions which might be useful:
1 - Being able to round exported values (e.g. rounding to 2 decimal places before writing the samples to the csv file might help reducing the final size of the file and improving predictability of the size required)
2 - It would be nice to have the possibility to export data to DBs (or at least SQLite) instead of CSV files. This could be done by offering a mechanism to specify a custom "Exporter" in the YML config (pretty much the same way we specify custom Extractors), so that one could extend it and offer its own implementation.

[DanySK]

Both proposals are interesting, I like them

[fedpet]

The "rounding" could just be a parameter of the CSV exporter

[DanySK]

Partially tackled by #871