Move ApplyBCKCorrection from WarpX header to anonymous namespace in WarpXFaceExtensions.cpp

[lucafedeli88]

ApplyBCKCorrection(int idim); is only used inside WarpXFaceExtensions.cpp, therefore, with the aim of simplifying the WarpX class, this PR moves it to an anonymous namespace inside WarpXFaceExtensions.cpp.

The following modifications are also introduced:

• idim is now passed to ApplyBCKCorrection as a template parameter (in order to pass is as a template parameter to ComputeSStab, see below)
• ApplyBCKCorrection called ComputeSStab which was a free function inside WarpXFaceExtensions.cpp, therefore the PR moves ComputeSStab to the anonymous namespace
• ComputeSStab now takes idim as a template parameter for performance reasons (avoid ifs inside the kernel)
• a wrapper of ComputeSStab taking idim as a non-template argument is also provided, in order to avoid extensive modifications in other parts of WarpXFaceExtensions.cpp
• instead of directly throwing exceptions, we now use WARPX_ALWAYS_ASSERT_WITH_MESSAGE
• a [[maybe_unused]] attribute is added for the non-template version of ComputeSStab to avoid unused function issues raised by AppleClang and clang-tidy