This MR slightly extends the support for more general iteration slices on the CUDA platform,
greatly extends the test suite for iteration slices on all targets,
and fixes some bugs found along the way.
Code Generator Configuration
Add special value AUTO to pystencils.config to model automatic behavior; no longer use None to mean "automatic" in specifying ghost layers
Add configuration option manual_launch_grid to disable automatic inference of the GPU launch grid size
Iteration Slices on GPU
Have the Cuda platform raise a warning if it can't figure out a launch grid because of dependencies between dimensions
Extend the JIT-compiled kernel object to allow manual specification of the launch grid, and enforce this if no grid size was inferred from the kernel
This now enables the iteration limits of faster coordinates to depend on the current counter value of slower coordinates;
e.g. triangular iteration patterns, red-black checkerboard iteration, ... (see test cases)
Documentation for these features will be added in a follow-up MR.
Bugfixes
Fix parsing of iteration slices that are negative integers
Fix a bug in the loop vectorizer where the trailing loop was only executed if the SIMD-loop had run for at least one iteration
Test Suite
Add pytest fixtures for available codegen targets, to simplify writing tests that should succeed on all hardware
Add extensive tests for common and more uncommon iteration slices on all targets
