Workers’ compensation — Compensable injuries — Causation — Judge of compensation claims erred in finding that employer performed a reasonable and timely investigation into the compensability of claimant’s low back condition where JCC failed to determine when that investigation should have begun — Evidence — Unauthorized treating physician — JCC abused its discretion in excluding portions of chiropractor’s testimony where chiropractor was not asked to express an opinion relative to medical causation, but was asked to describe the treatment he provided to claimant and to put claimant’s complaints and clinical exam results into context