(Closes #1832) add support for force option to ParallelLoopTrans

[arporter]

Functionally, this is a small PR that just adds a 'force' option to ParallelLoopTrans. However, I've also take the opportunity to improve the testing of this class.

[codecov[bot]]

Codecov Report

Base: 99.83% // Head: 99.84% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (1020510) compared to base (cacca74). Patch coverage: 100.00% of modified lines in pull request are covered.

:exclamation: Current head 1020510 differs from pull request most recent head 192e5ec. Consider uploading reports for the commit 192e5ec to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1925    +/-   ##
========================================
  Coverage   99.83%   99.84%            
========================================
  Files         302      301     -1     
  Lines       41840    41659   -181     
========================================
- Hits        41773    41594   -179     
+ Misses         67       65     -2     

Impacted Files	Coverage Δ
src/psyclone/core/component_indices.py	100.00% <ø> (ø)
...clone/psyir/transformations/parallel_loop_trans.py	100.00% <100.00%> (ø)
src/psyclone/transformations.py	99.36% <100.00%> (-0.02%)	:arrow_down:
src/psyclone/dynamo0p3.py	99.88% <0.00%> (-0.01%)	:arrow_down:
src/psyclone/nemo.py	100.00% <0.00%> (ø)
src/psyclone/psyir/transformations/__init__.py	100.00% <0.00%> (ø)
...psyclone/psyir/transformations/acc_update_trans.py
src/psyclone/gocean1p0.py	99.86% <0.00%> (+<0.01%)	:arrow_up:
src/psyclone/psyir/nodes/acc_directives.py	100.00% <0.00%> (+0.36%)	:arrow_up:

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[arporter]

Note that I'm not sure how best to document this new option. It's in the class docstring but it's a virtual class that is then sub-classed by several different transformations. Adding it to the docstring of each of those doesn't feel like the right thing to do but I'm open to suggestions.

Ready for review from @sergisiso, @rupertford or @hiker .

[arporter]

Ready for another look now.