api: Add shift and fd order option to all FD operators:

[mloubout]

Add options so can do things like div(u, order=so, shift=.5) for left/right laplacian

[codecov]

Codecov Report

Merging #2258 (8b1ce77) into master (39a8486) will increase coverage by 0.05%.
The diff coverage is 97.85%.

@@            Coverage Diff             @@
##           master    #2258      +/-   ##
==========================================
+ Coverage   86.88%   86.94%   +0.05%     
==========================================
  Files         229      229              
  Lines       42033    42109      +76     
  Branches     7760     7777      +17     
==========================================
+ Hits        36521    36612      +91     
+ Misses       4871     4854      -17     
- Partials      641      643       +2     

Files	Coverage Δ
devito/finite_differences/differentiable.py	94.27% <100.00%> (+0.06%)	⬆️
tests/test_derivatives.py	100.00% <100.00%> (ø)
tests/test_tensors.py	95.56% <100.00%> (+0.87%)	⬆️
devito/finite_differences/operators.py	64.28% <87.50%> (+7.14%)	⬆️
devito/types/tensor.py	92.59% <93.33%> (+7.09%)	⬆️

[FabioLuporini]

Uncontroversial, thanks