Standardized the selection of required and optional fields in BCs, fvOptions, functionObjects etc.

In most boundary conditions, fvOptions etc. required and optional fields
to be looked-up from the objectRegistry are selected by setting the
keyword corresponding to the standard field name in the BC etc. to the
appropriate name in the objectRegistry.  Usually a default is provided
with sets the field name to the keyword name, e.g. in the
totalPressureFvPatchScalarField the velocity is selected by setting the
keyword 'U' to the appropriate name which defaults to 'U':

        Property     | Description             | Required    | Default value
        U            | velocity field name     | no          | U
        phi          | flux field name         | no          | phi
        .
        .
        .

However, in some BCs and functionObjects and many fvOptions another
convention is used in which the field name keyword is appended by 'Name'
e.g.

        Property     | Description             | Required    | Default value
        pName        | pressure field name     | no          | p
        UName        | velocity field name     | no          | U

This difference in convention is unnecessary and confusing, hinders code
and dictionary reuse and complicates code maintenance.  In this commit
the appended 'Name' is removed from the field selection keywords
standardizing OpenFOAM on the first convention above.