revise: MeasuredQuantity vs MeasuredSignalType vs ComputedDataSignature (new?)

[lzehl]

@openMetadataInitiative/openminds-developers to be discussed (@apdavison in particular with you)

• related to request Add experimentalApproach/functionalConnectivity #485
• related also to all neuroimaging maps
• related also to LFP term suggestion

[tgbugs]

Summary of discussion.

Measured quantity is the thing that you are trying to measure in principle, e.g. membrane potential. Measured signal type is what comes off the the machine and is therefore a relationship between a dataset and a data type or file and not a superclass because e.g. spike times could be both a measured signal type and a computed data signature depending on whether the machine doing the digitization produced a voltage time series or a set of spike times from a reference moment.