TODO: migrate singleCellFvMesh into simplified mesh framework (?)

Merge branch 'develop-pre-release' of develop.openfoam.com:Development/OpenFOAM-plus into develop-pre-release

- samples on cell centres, can optionally limit based on a bounding box.
  Can be used, for example, to extract volume fields into CSV format.

- add some more documentation

Merge branch 'develop-pre-release' of develop.openfoam.com:Development/OpenFOAM-plus into develop-pre-release

- improve handling of model: ... line

- simplify parsing of xxx element: ... lines

- start/end values were underrepresented due to rounding.
  Now extend the range to include -0.5 and +0.5 beyond the usual
  range to ensure the same number density.

Merge branch 'develop-pre-release' of develop.openfoam.com:Development/OpenFOAM-plus into develop-pre-release

Merge branch 'develop-pre-release' of develop.openfoam.com:Development/OpenFOAM-plus into develop-pre-release

See:
src/rigidBodyDynamics/restraints/prescribedRotation
tutorials/multiphase/overInterDyMFoam/boatAndPropeller

Fields now correspond to the initial residual for the first solver
iteration

Residual fields can be written using the new 'writeFields' entry, e.g.

    functions
    {
        residual
        {
            type            residuals;
            libs            ("libutilityFunctionObjects.so");
            fields          (".*");
            writeControl    writeTime;
            writeFields     true;
        }
    }

Fields currently correspond to the initial residual for the last solver
iteration.