Verify step <> start arithmetic

[Erikvv]

Before the time series is used the constructor or builder should verify that the combination of start and step works.

Can verify alignment too.

[GillisHommen]

What does alignment mean in this context? That the requested sampling-points are 'exactly' on the steps of the dataset?

[Erikvv]

For example if a time series starts at T00:05:00 but has a step of PT15M that seems off because the start time is not a multiple of the step.