IOlevel TTL on Wavedump 2 causes `process_header()` to break

[jwaiton]

For some reason, the TTL setting on WaveDump 2 makes it such that the event numbers increment by 2 rather than 1. This breaks our header checker code as it expected singular increments.

A simple fix that I've applied locally is to remove the event_number_1 - 1 = event_number requirement for the header checker, as having a correct sample rate and period should be enough to validate the header. This needs to be applied to MULE and tested.