Added improved merging of staging dicts

[wk9874]

Added deepmerge as a dependency, wrote a custom merge strategy which merges dicts and overrides lists, and replaced _local_data | self._staging with the deep merge.

Chose to override lists as the deduplication for alerts and tags is handled by the run, so dont need to append them. Also decided with Andrew that for metadata it would be better to overwrite, eg:

run.update_metadata({"test": [1,2]})
run.update_metadata({"test": [3,4]})

Should return {"test": [3,4]}, not {"test": [1,2,3,4]}

Closes #691