Use start and end intervals, rather than history and future

[peterdudfield]

Detailed Description

Use start and end intervals, rather than history and future

Context

• currently using history and future minutes
• will help to clarify about if it includes t0 or not - see Document what t0 is ocf-data-sampler#326

Possible Implementation

• change in ocf-data-sampler
• change in pvnet + migration script for HF

[dfulu]

@peterdudfield I think this is for PVNet not data-sampler