Pymartini

- Remove `column_row` option of `rescale_positions`; when `False` it created
incorrect meshes by using a transposed mesh for the mesh creation and height
lookup steps. See 15.

- Add `column_row` argument to `rescale_positions`, to use the right axes when
looking up terrain values.
- Convert `rescale_positions` to python/numpy instead of cython, for ease of
maintainability. This does make `rescale_positions` a tiny bit slower, but
it's not a perceptible difference. Rescaling 7,000 vertices was 40 μs with the
cython code and 190 μs with the python/numpy code.

- Allow ndarray as input to `Martini.create_tile`

- Allow `decode_ele` to take arrays of transposed shapes, and add an option to
turn off backfilling

- Test building wheels with Github actions

- Convert Python code to Cython
- Add utilities to extract terrain from images and to rescale positions
- Add benchmarks
- Add tests, explain rounding errors between `pymartini` and Martini in README.