Use constexpr std::string_view

[BenWibking]

Replaces const std::string usage with constexpr std::string_view.

[BenWibking]

The Castro CI failure looks unrelated:

../../../Source/gravity/Gravity.cpp: In member function ‘amrex::Real Gravity::actual_solve_with_mlmg(int, int, const amrex::Vector<amrex::MultiFab*>&, const amrex::Vector<const amrex::MultiFab*>&, const amrex::Vector<std::array<amrex::MultiFab*, 2>, std::allocator<std::array<amrex::MultiFab*, 2> > >&, const amrex::Vector<amrex::MultiFab*>&, const amrex::MultiFab*, amrex::Real, amrex::Real) const’:
../../../Source/gravity/Gravity.cpp:3777:18: error: ‘using amrex::MLMG = class amrex::MLMGT<amrex::MultiFab>’ {aka ‘class amrex::MLMGT<amrex::MultiFab>’} has no member named ‘setConvergenceNormType’
 3777 |             mlmg.setConvergenceNormType(MLMGNormType::bnorm);
      |                  ^~~~~~~~~~~~~~~~~~~~~~
../../../Source/gravity/Gravity.cpp:3777:41: error: ‘MLMGNormType’ has not been declared
 3777 |             mlmg.setConvergenceNormType(MLMGNormType::bnorm);
      |                                         ^~~~~~~~~~~~
make: *** [/home/runner/work/Microphysics/Microphysics/amrex/Tools/GNUMake/Make.rules:260: tmp_build_dir/o/2d.gnu.EXE/Gravity.o] Error 1

[zingale]

yes, that is unrelated. It will go away once we merge development in Castro.