Allow for upsampling of instantanious measurements

[nhoening]

Flexmeasures upsamples data sent to the API in a lower resolution which is a multiple of the required sensor resolution. This doesn't apply to sensor measuring instantaneous values.

This requires at least a small fix in the validator post_data_checked_for_required_resolution.

There is a bigger modelling topic of sensor resolution vs data frequency behind this, but in this issue we won't tackle all of it.