Record history in the experiment list

[dagewa]

This adds history tracking to ExperimentList, such that every time an experiment list is written to disk, a history item is added containing the name of the module that called as_file (or as_json) and a UTC timestamp.

This is a step towards improved export of history to MTZ for dials/dials#2861.

Unfortunately it turns out that in DIALS we almost always create a new ExperimentList and copy experiments into it rather than modifying the list loaded into a program, so the change here is not enough to preserve all history items. This PR will have to be followed by a DIALS PR that ensures history is also copied when we make new ExperimentList objects.

[dagewa]

I might rethink this. The motif of making a new ExperimentList and copying experiments into it is prevalent in DIALS. Plus we support various workflows that include combining and splitting of experiment lists. At the moment, the top-level history keeps getting lost and I'd have to make a lot of DIALS changes to copy history back in from the input prior to saving. It may be better to record a separate history per experiment. That would help preserve it through all sorts of wrangling and would correctly keep track in the case of confluence of experiments from different processing chains.

[dagewa]

Superseded by #814