Set SYCL subgroup size via kernel property or @simd_length attribute.

[kris-rowe]

Description

• Introduces the @simd_length attribute and kernel property
• For dpcpp mode these can be used to prescribe which subgroup size will be used
• OpenCL and OpenMP modes could make use of this property in the future

Closes #693

[codecov]

Codecov Report

Merging #726 (996bd56) into development (8957558) will decrease coverage by 0.05%.
The diff coverage is 70.42%.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #726      +/-   ##
===============================================
- Coverage        75.26%   75.22%   -0.05%     
===============================================
  Files              297      299       +2     
  Lines            19327    19387      +60     
===============================================
+ Hits             14546    14583      +37     
- Misses            4781     4804      +23     

Files	Coverage Δ
src/occa/internal/lang/attribute.cpp	69.82% <100.00%> (+0.80%)	⬆️
...a/internal/lang/builtins/attributes/simdLength.hpp	100.00% <100.00%> (ø)
src/occa/internal/lang/modes/dpcpp.hpp	100.00% <ø> (ø)
src/occa/internal/lang/modes/okl.cpp	93.67% <100.00%> (+0.03%)	⬆️
src/occa/internal/lang/modes/withLauncher.cpp	94.24% <100.00%> (+0.03%)	⬆️
...a/internal/lang/builtins/attributes/simdLength.cpp	50.00% <50.00%> (ø)
src/occa/internal/lang/modes/dpcpp.cpp	80.42% <75.00%> (-2.48%)	⬇️

... and 2 files with indirect coverage changes