File format: unit=bytes makes startValue/endValue meaningless

When unit=milliseconds, it's clear that it applies to both timestamps (like startValue) and to sample weights.

But when unit=bytes, there's no way to know what startValue/endValue are supposed to mean.

We should consider separate timeUnit and weightUnit fields.

The issue does not name a file or test. Start by tracing how unit, startValue, endValue, and sample weights are represented in the file format; done means reaching a decided approach for separating timeUnit and weightUnit.

Written by the indexing model from the issue text.