ENH: outletMappedUniformInlet: add optional fraction and offset entries

The new functionality optionally allows the patch-averaged
value to be scaled and/or offset by a pair of specified values.

Example of the boundary condition specification:

```
<patchName>
{
    // Mandatory entries (unmodifiable)
    type            outletMappedFilterInlet;
    outletPatch     <outletPatchName>;

    // Optional entries (unmodifiable)
    fraction        0.1;
    offset          10;    // (1 0 0);
    phi             phi;

    // Optional (inherited) entries
    ...
}
```