Reported changes when nothing is changed

[niekdejonge]

When rerunning a pipeline I would get changes in the report even though nothing was changed, this was the case for: add_retention_index, add_retention_time and add_parent_mass. For the retention time, this happened because missing metadata was set to None. And that is detected as a change, because it was not there at all at first. This should be changed in the detection, if the field is missing before and is set to None after, it should not be counted as a change.
For the parent mass I think it is due to the parent mass being changed from str to float. For other fields this happens, when loading in (in Metadata class), but for parent mass it doesn't. Maybe we should make a differentiation between functions that reformat to computer readable formats (part of loading) and actual cleaning.