plan_* functions should warn when an invalid plan is generated

[kleinschmidt]

Currently plan_edf_to_onda_samples will happily produce a plan with e.g. duplicated channel names, without any warning, but executing this plan produces an error. We should check that the plan will produce valid SamplesInfo at the very least. Probably also would be good to allow users to silence these warnings in the case of datasets with many recordings that have issues that need to be corrected after the plan is generated.