ENH: runTime calculation of zeroGradient volume fields (issue #235)

Extrapolate internal field to walls for post-processing.
Uses as new syntax for handling the naming of multiple fields.

The input fields are selected via a wordReList.
For example,

    fields      (U "(T|k|epsilon|omega)");

The names of the resulting output fields use placeholder tokens for
flexibility. For example,

    result      zeroGradient(@@);

The '@@' placeholder is replaced by the name of the input field.
Eg,
    fields      (U T);
    result      zeroGradient(@@);
->  zeroGradient(U), zeroGradient(T)

Or,
    fields      (U T);
    result      @@nearWall;
->  UnearWall, TnearWall

NOTE:
    The function object will skip over fields that only have
    processor, empty, zeroGradient patches. The operation does not
    make much sense for these, and it avoids inadvertently
    re-processing fields twice.