BUG: forces: ensure UName entry can be used (fixes #2093)

Previously, for basic incompressible and compressible simulations,
the "force" function object has not been using the user-specified "UName"
for the "devRhoReff" computation (affecting the tangential component),
but using the "U" of the latest available step. In contrast,
the user-specified "pName" has always been being used correctly.

This has been causing issues for users when they wish to use a specific
"UMean" field in various force and forceCoeff function object computations.