Adjust nomenclature for "epoch"s

[geoffxy]

In most parts of the BRAD codebase, "epoch" is now the "sampling period" (for metrics, workload logs). We probably still want something like an epoch for forecasting and planning, where this value is greater than the sampling period.

These two values should be decoupled to enable finer grained decisions, especially for our experiments. We do not want to run for more than a few minutes at a time. Thus we want to sample metrics at one frequency and then run forecasts and planning over multiple samples.