Add RecordEvent message for provenance

[skearnes]

Fixes #44

Also changes id to record_id and makes it a string field for flexibility (people shouldn't be typing record IDs by hand anyway).

[connorcoley]

For uniformity, would it make sense to also make experiment_start used this new RecordEvent message?

We have both a Person and DateTime associated with the provenance:

Person experimenter = 1;
DateTime experiment_start = 3;

I don't necessarily believe this is a good idea, but I wanted to bring it up as something to consider if you hadn't already. I think I prefer the current solution of having the experimenter and start time being direct fields within ReactionProvenance.

[skearnes]

For uniformity, would it make sense to also make experiment_start used this new RecordEvent message?

We have both a Person and DateTime associated with the provenance:

Person experimenter = 1;
DateTime experiment_start = 3;

I don't necessarily believe this is a good idea, but I wanted to bring it up as something to consider if you hadn't already. I think I prefer the current solution of having the experimenter and start time being direct fields within ReactionProvenance.

I considered this and also preferred the direct field approach.

[skearnes]

Also, here's another link describing why required was discouraged for proto2: https://stackoverflow.com/a/31814967