naming inconsistency (UName, pName, ...)

Previously we had "UName", "pName"..., which have mostly be changed to "U", "p" .. for dictionary inputs.

Ones still using the older names:

• solvers/basic/potentialFoam/createFields.H
• solvers/basic/potentialFoam/overPotentialFoam/createFields.H
• solvers/basic/potentialFoam/overPotentialFoam/overPotentialFoam.C
• solvers/basic/potentialFoam/potentialFoam.C
• fvOptions/constraints/derived/velocityDampingConstraint/velocityDampingConstraint.C
• fvOptions/sources/derived/acousticDampingSource/acousticDampingSource.C