Ideally wish to delay construction of the finite-area mesh until
updateCoeffs(), when it is actually needed.
This helps avoid difficult to trace errors and avoids inadvertent
parallel communication during construction from a dictionary.
However, lazy evaluation fails at the later stage while attempting
to load the corresponding initial fields, since the timeIndex has
already advanced.
Use the newly introduced regionModels::allowFaModels() switch
to locally enable or disable lazy evaluation as needed.
This allows selective disabling (eg, in post-processing utilities)
where loading the volume field should not activate the associated
regionFaModel and loading a finite-area mesh too (which may not
exist or be defined at that point).
ac34d9fd29