Pympdata

- addressing compatibility with Numba 0.55 (incl. elimination of read-only globals for null arrays)
- refactor around passing arrays & boundary conditions in Stepper (using namedtuple)

- docstring updates
- fixes in Julia and Matlab snippets in README
- switch from exdown to pytest-codeblocks in CI workflows (package renamed)
- indicate incompatibility with Numba 0.55 in setup.py

- cleanups & refactors: first release for which pylint report zero issues
- pylint check is now part of CI workflow (failure is any issue raised)
- docstring updated (also checked by pylint)
- figures in README: grid layout in 2D and wireframe pltos inspired by Fig. in Arabas et al. 2014 (Sci. Prog.)
- 3d timing tests
- smoke tests for the 3D example against libmpdata++ reference data

- asserts vs. analytic solution in shallow-water smoke test

- new smoke test based on the shallow-water example