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This is implemented for one field.

This is to implement catching of math domain errors, before they occur.

These tests cover all parameter value combinations which result in no exchange:

- dw = 0.0,
- pA = 1.0,
- kex = 0.0,
- dw = 0.0 and pA = 1.0,
- dw = 0.0 and kex = 0.0,
- pA = 1.0 and kex = 0.0,
- dw = 0.0, pA = 1.0, and kex = 0.0.
- kex = 1e5,

Task #7807 (https://gna.org/task/index.php?7807): Speed-up of dispersion models for Clustered analysis.