Rationalize data validation into a single location

Currently there is data validation happening at multiple layers. For example the function `DataloggerMainWindow.handle_instrument_changed()` currently passes the raw data from the GUI widgets down into lower layers, and then there are multiple conversions and checks done at various levels. All of these checks can probably be hoisted into the GUI layer, which can populate a (new?) data structure containing the relevant information to pass to the backend for processing.

This was noted as part of the review of PR #11.