Blurb::
Add context to data: number of configuration variables.

Description::
If there are multiple experiments, there can be different
configuration variables (e.g. experimental settings, boundary
conditions, etc.) per experiment.  See \ref
responses-calibration_terms-calibration_data or \ref
responses-calibration_terms-calibration_data_file.

During calibration, configuration variables are state variables which
will be passed to the simulation, and are not treated as calibration
parameters.

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