Detect & fail sample set on duplicate ERT times before downlink data rate calculations

[rmseng]

Duplicate frames (e.g. in a test environment) can cause a divide by zero error during downlink data rate estimate, if that functionality is in use, in the case where subsequent frames (after sorting) have the same ERTs. This is a confusing error to log in this case, and our users would be better served by an obvious error stating that the sample set has frames with the same ERTs, and that the sample set was being discarded.

Make changes so that MMTC validates a sample set on having unique ERTs before downlink data rate is calculated and set.