PVDataSource should only correct PVOutput.org timezone on "legacy" PVOutput.org data

[JackKelly]

Since PRs openclimatefix/pvoutput#102 and openclimatefix/PVConsumer#11, our PVConsumer code will convert datetimes to UTC. But nowcasting_dataset assumes that PVOutput.org data is in Europe/London timezone (see these two lines of code in nowcasting_dataset). Somehow, nowcasting_dataset needs to know if the PVOutput.org is already in UTC, or if it still needs to be converted from localtime to UTC.

Context

• Convert downloaded data from localtime to UTC  pvoutput#14
• add optional for timezone in status pvoutput#102
• add timezone PVConsumer#11
• Add timezone to data PVConsumer#8

Implementation

Perhaps the easiest solution might be to convert the PVOutput.org data we currently have on disk to UTC, and then remove these two lines from nowcasting_dataset.

Or, perhaps a better solution would be to modify pvoutput so it explicitly sets the timezone to UTC on all the datetimes (see openclimatefix/pvoutput#14 (comment)). Then nowcasting_dataset would only assume PVOutput.org data needs converting from Europe/London timezone to UTC if the timestamps lack an explicit UTC offset.

[peterdudfield]

perhaps #563 could be done first