Roadmap to v5.0

[ZedThree]

Here's what I see as the remaining tasks to be done before releasing v5.0, with what order things need to be done in.

Coordinates

1. Finish and merge 3D metrics (3D metrics #2025)
2. Refactor coordinates (Clean up Coordinates() constructor #2128)
3. Add real-space coordinate values to Hypnotoad and Zoidberg output (possibly already exists?)
   • Possibly combine Hypnotoad and Zoidberg? Less important for BOUT++ v5
4. Add Z guard cells and allow non-periodic Z (Start to introduce zstart/zend and change loops (updated) #1713 )
5. Refactor boundary conditions (proof of concept in ShiftedMetric with 2 yup/ydown points #1178)

IO

1. Replace DataFile with OptionsNetCDF (Replace DataFile/Dataformat with OptionsNetCDF #2209)
2. Replace Mesh::get and GridDataSource with OptionsNetCDF

Other

• Remove non-PhysicsModel API for Solver (Remove legacy Solver API #2285)
• Merge all the upgrader python scripts into a single utility
• Split index_derivs.cxx into multiple files to improve compilation time (and make it work on more compilers...) (Split index_derivs into two files #2292)
• Consider upgrading to C++17? Less important
• Start to use xBOUT in the library? Less important
• FFT improvements? Potential future performance issue with multiple Meshes and FFTW #2097
• CMake feature parity