feat: add eval option to sample.getExpectation() showing param modified yields

[andrzejnovak]

• Add option to call sample.getExpectation(eval=True) which return yields similarly to sample.getExpectation(nominal=True) but takes into account the current values of the assigned nuisances
• Force a copy when passing templates to TemplateSample to prevent user objects being modified
• Add warnings against negative yields in templates and force_positive=True convenience option to set negative values to 0.
• raise errors when trying to add multiple nuisances with the same name to a channel
• emit warnings when rendendering to combine if multiple nuisances with the same name are still present in the model (eg when they were added to different channels)

@nsmith-

[andrzejnovak]

@nsmith- anything missing here?