operator: Expose more opt knobs to the user level

[FabioLuporini]

No description provided.

[codecov]

Codecov Report

Merging #1387 into master will decrease coverage by 0.00%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #1387      +/-   ##
==========================================
- Coverage   87.25%   87.25%   -0.01%     
==========================================
  Files         186      186              
  Lines       26784    26803      +19     
  Branches     3638     3641       +3     
==========================================
+ Hits        23371    23386      +15     
- Misses       2988     2991       +3     
- Partials      425      426       +1     

Impacted Files	Coverage Δ
devito/core/gpu_openacc.py	44.30% <33.33%> (ø)
devito/core/gpu_openmp.py	82.14% <85.00%> (-0.02%)	⬇️
tests/test_operator.py	97.75% <88.88%> (-0.18%)	⬇️
devito/passes/iet/openmp.py	95.81% <91.66%> (ø)
devito/core/cpu.py	98.96% <100.00%> (+1.24%)	⬆️
devito/operator/operator.py	87.69% <100.00%> (-0.57%)	⬇️
tests/test_autotuner.py	100.00% <100.00%> (ø)
tests/test_dle.py	97.51% <100.00%> (-0.12%)	⬇️
tests/test_mpi.py	93.08% <100.00%> (-0.02%)	⬇️
... and 1 more

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[mloubout]

Looks super nice!!!