ckp: Checkpointing update

[navjotk]

1. Starts to cache prepared arguments on the CheckpointOperator object to reduce the overhead of argument processing every time the operator is called.
2. Moves checkpointing inside the main Devito folder.

[codecov]

Codecov Report

Merging #1774 (6fa82e3) into master (491baef) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1774      +/-   ##
==========================================
- Coverage   89.47%   89.45%   -0.02%     
==========================================
  Files         208      209       +1     
  Lines       33932    33934       +2     
  Branches     4408     4404       -4     
==========================================
- Hits        30361    30357       -4     
- Misses       3074     3081       +7     
+ Partials      497      496       -1     

Impacted Files	Coverage Δ
devito/checkpointing/__init__.py	100.00% <ø> (ø)
devito/__init__.py	93.75% <100.00%> (+0.09%)	⬆️
devito/checkpointing/checkpoint.py	100.00% <100.00%> (ø)
examples/seismic/acoustic/wavesolver.py	100.00% <100.00%> (ø)
examples/seismic/tti/wavesolver.py	98.33% <100.00%> (-0.02%)	⬇️
examples/seismic/viscoacoustic/wavesolver.py	100.00% <100.00%> (ø)
tests/test_checkpointing.py	100.00% <100.00%> (ø)
devito/arch/archinfo.py	42.89% <0.00%> (-4.44%)	⬇️
devito/passes/iet/engine.py	91.52% <0.00%> (-3.89%)	⬇️
devito/arch/compiler.py	54.31% <0.00%> (-2.56%)	⬇️
... and 18 more

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[FabioLuporini]

we should add CheckpointOperator to the user API by adding it to devito/__init__.py

[navjotk]

I have now added the two classes to the user-facing API and updated all examples. I tried to squash the commits but that merge in the middle sent me into git hell for a bit. I quickly reverted and gave up on giving you a clean commit history :)