Write 2D plotfiles for projections (#791)

### Description
Writes projections as 2D plotfiles, rather than as 3D plotfiles with one
cell in the 3rd dimension.

### Related issues
Fixes #685.

### Checklist
_Before this pull request can be reviewed, all of these tasks should be
completed. Denote completed tasks with an `x` inside the square brackets
`[ ]` in the Markdown source below:_
- [x] I have added a description (see above).
- [x] I have added a link to any related issues see (see above).
- [x] I have read the [Contributing
Guide](https://github.com/quokka-astro/quokka/blob/development/CONTRIBUTING.md).
- [ ] I have added tests for any new physics that this PR adds to the
code.
- [x] I have tested this PR on my local computer and all tests pass.
- [x] I have manually triggered the GPU tests with the magic comment
`/azp run`.
- [x] I have requested a reviewer for this PR.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/CMakeLists.txt

@@ -134,6 +134,7 @@ set (QuokkaSourcesNoEOS "${CMAKE_CURRENT_SOURCE_DIR}/main.cpp"
                         "${CMAKE_CURRENT_SOURCE_DIR}/io/DiagFilter.cpp" 
                         "${CMAKE_CURRENT_SOURCE_DIR}/io/DiagFramePlane.cpp" 
                         "${CMAKE_CURRENT_SOURCE_DIR}/io/DiagPDF.cpp" 
+                        "${CMAKE_CURRENT_SOURCE_DIR}/io/projection.cpp" 
                         "${CMAKE_CURRENT_SOURCE_DIR}/cooling/GrackleLikeCooling.cpp" 
                         "${CMAKE_CURRENT_SOURCE_DIR}/cooling/GrackleDataReader.cpp"
                         "${CMAKE_CURRENT_SOURCE_DIR}/cooling/TabulatedCooling.cpp" 

src/QuokkaSimulation.hpp

@@ -190,7 +190,7 @@ template <typename problem_t> class QuokkaSimulation : public AMRSimulation<prob
 	void ComputeDerivedVar(int lev, std::string const &dname, amrex::MultiFab &mf, int ncomp) const override;
 
 	// compute projected vars
-	[[nodiscard]] auto ComputeProjections(int dir) const -> std::unordered_map<std::string, amrex::BaseFab<amrex::Real>> override;
+	[[nodiscard]] auto ComputeProjections(const amrex::Direction dir) const -> std::unordered_map<std::string, amrex::BaseFab<amrex::Real>> override;
 
 	// compute statistics
 	auto ComputeStatistics() -> std::map<std::string, amrex::Real> override;
@@ -600,7 +600,7 @@ template <typename problem_t> void QuokkaSimulation<problem_t>::ComputeDerivedVa
 }
 
 template <typename problem_t>
-auto QuokkaSimulation<problem_t>::ComputeProjections(int /*dir*/) const -> std::unordered_map<std::string, amrex::BaseFab<amrex::Real>>
+auto QuokkaSimulation<problem_t>::ComputeProjections(const amrex::Direction /*dir*/) const -> std::unordered_map<std::string, amrex::BaseFab<amrex::Real>>
 {
 	// compute projections and return as unordered_map -- user should implement
 	return std::unordered_map<std::string, amrex::BaseFab<amrex::Real>>{};