Avoid ambiguous type conversion when accessing tmp<vectorField>

[MakisH]

Reported by a user using the Intel compiler (OneAPI 2023.2, RHEL 8, OpenFOAM v2406) in the forum.

Resolves errors such as:

In file included from FF/ModuleFF.C:3:
FF/Velocity.C:95:25: error: conversion from 'tmp<vectorField>' (aka 'tmp<Field<Vector<double>>>') to 'vectorField' (aka 'Field<Vector<double>>') is ambiguous
            vectorField n = U_->boundaryField()[patchID].patch().nf();
                        ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/leonardo/prod/spack/05/install/0.21/linux-rhel8-icelake/oneapi-2023.2.0/openfoam-2406-psbdrns5bo35czf4hemtwyre65rfdf32/src/OpenFOAM/lnInclude/tmp.H:320:9: note: candidate function
        operator const T&() const { return cref(); }
        ^
/leonardo/prod/spack/05/install/0.21/linux-rhel8-icelake/oneapi-2023.2.0/openfoam-2406-psbdrns5bo35czf4hemtwyre65rfdf32/src/OpenFOAM/lnInclude/Field.H:266:16: note: candidate constructor
        inline Field(const tmp<Field<Type>>& tfld);
               ^

[MakisH]

Closing for technical reasons (running system tests in a fork currently not provided). Opening a new PR.

[precice-bot]

This pull request has been mentioned on preCICE Forum on Discourse. There might be relevant details there:

https://precice.discourse.group/t/failing-to-compile-the-openfoam-adapter-at-moduleff-o/2240/5