make Image and Measurement creation atomic together

[marxide]

When ingesting inputs for a pipeline run, an Image object is created and saved to the database, then the catalogue is read, its contents are saved to the database as Measurement objects, and a copy of the catalogue is saved to the filesystem in a parquet file.

This PR puts the Image and Measurement object creation in an atomic transaction, i.e. if one fails, all operations conducted during the transaction are undone. If the catalogue read fails, the Image will be deleted from the database.

Fixes #647

[ajstewart]

Is the sky region, if it was a new one, also dumped if the image fails? Can't quite see it from that changed code, though maybe it's not even created there.

Is this something that should be included in testing?

[marxide]

Is the sky region, if it was a new one, also dumped if the image fails? Can't quite see it from that changed code, though maybe it's not even created there.

The sky regions are retrieved/created along with the image, so it should be within the same atomic transaction, just buried elsewhere.

Is this something that should be included in testing?

Good point. I've added one in 05adcf7. While I was there I also added a tearDown for RunPipelineTest that cleans up the run products so the pipeline-run folders are back in their original state after the tests are complete.