omegaWallFunction: Improved low-Reynolds number behavior and consistency with the epsilonWallFunction

Changed default mode of operation to use standard y+ based switching
rather than the previous ad hoc blending and added consistent handling
of the near-wall generation term.

This boundary condition provides a wall constraint on turbulnce specific
dissipation, omega for both low and high Reynolds number turbulence models.

The near-wall omega may be either blended between the viscous region and
logarithmic region values using:

    \f[
        \omega = sqrt(\omega_{vis}^2 + \omega_{log}^2)
    \f]

where

\vartable
    \omega_{vis} | omega in viscous region
    \omega_{log} | omega in logarithmic region
\endvartable

see eq.(15) of:
\verbatim
    Menter, F., Esch, T.
    "Elements of Industrial Heat Transfer Prediction"
    16th Brazilian Congress of Mechanical Engineering (COBEM),
    Nov. 2001
\endverbatim

or switched between these values based on the laminar-to-turbulent y+ value
derived from kappa and E.  Recent tests have shown that the standard
switching method provides more accurate results for 10 < y+ < 30 when used
with high Reynolds number wall-functions and both methods provide accurate
results when used with continuous wall-functions.  Based on this the
standard switching method is used by default.