Add an option to use absolute tolerance

Issue/feature addressed

When using an iterative solver, the absolute tolerance is more useful to supress numerical oscilaltions in the force history. Since force is directly correlated with the absolute values of pressure (p) and velocity (0.5 rho U^2), oscillations in the force should have the order of tolerance of pressure, or U multiplies the tolerance of velocity.

Proposed solution

A bool-type variable is introduced to indicate if the tolerance is an absolute tolerance.

Suggested reviewers

Please suggest any people who would be appropriate to review your code. Parv, Alexandra

Notes

Please add any other information that could be useful for reviewers.

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).