Change clone method to explicitly set fields rather than use copy.copy

[jaredlockhart]

Presently, we copy the instance in memory with all of its fields and then explicitly set some to None that should not be set on the clone. However this results in newly added fields being unintentionally copied and creating bugs, which we've seen several times now. We should switch the clone method to make an empty experiment instance and then explicitly set all the fields we want to clone and then when we add new fields that we do want to clone we can explicitly add them there.

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