Previously, a nutWallFunctionFvPatchScalarField ref should be
created in epsilon, k, and omega wall functions to fetch various
common wall-function coefficients necessary to carry out and complete
local operations inside these wall functions.

However, this arrangement required the use of a nut wall function,
even when unnecessary, when any of non-nut wall functions are being used.
Therefore, some users had been redundantly restrained and
obstructed with rather obscure casting-error messages.

Also, the wall-function coefficients Cmu, kappa and E have been obtained
from the specified nutWallFunction in order to ensure that each patch
possesses the same set of values for these coefficients.

Although the motivation sounds reasonable, it has also been putting redundant
restraints on users and disregarding the specifics of each wall-function.
For example, the variation of epsilon in near-wall regions is usually very
steep and non-monotonic specific - an expert user may therefore want to use
an epsilon-specific coefficient, and this was not allowed by the previous
arrangement.

This commit introduces a new class (i.e. wallFunctionCoefficients) comprising
all common wall-function coefficients and yPlus calculations.