ENH: fixedMeanOutletInletFvPatchField: New outlet/inlet boundary condition...

ENH: fixedMeanOutletInletFvPatchField: New outlet/inlet boundary condition which fixes the outlet mean

Description
    This boundary condition extrapolates field to the patch using the near-cell
    values and adjusts the distribution to match the specified, optionally
    time-varying, mean value.  This extrapolated field is applied as a
    fixedValue for outflow faces but zeroGradient is applied to inflow faces.

    This boundary condition can be applied to pressure when inletOutlet is
    applied to the velocity so that a zeroGradient condition is applied to the
    pressure at inflow faces where the velocity is specified to avoid an
    unphysical over-specification of the set of boundary conditions.

Usage
    \table
        Property     | Description             | Required    | Default value
        meanValue    | mean value Function1    | yes         |
        phi          | Flux field name         | no          | phi
    \endtable

    Example of the boundary condition specification:
    \verbatim
    <patchName>
    {
        type            fixedMeanOutletInlet;
        meanValue       1.0;
    }
    \endverbatim

See also
    Foam::fixedMeanFvPatchField
    Foam::outletInletFvPatchField
    Foam::Function1Types