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 (#2025)
2. Refactor coordinates (#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 (#1713 )
5. Refactor boundary conditions (proof of concept in #1178)

IO

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

Other

• Remove non-PhysicsModel API for Solver (#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...) (#2292)
• Consider upgrading to C++17? Less important
• Start to use xBOUT in the library? Less important
• FFT improvements? #2097
• CMake feature parity