(Closes #1903) rationalise OMP schedule values

[arporter]

Changes the default OMP schedule to be "none" which means that no schedule clause is added to the OpenMP directive. Also refactors things slightly so that the OMPScheduleClause is the single source of truth for permitted values of schedule.

[arporter]

Note to self: check testing of OMPParallelDoDirective.gen_code as test for schedule rather than `schedule != "none" wasn't picked up.

[codecov[bot]]

Codecov Report

Base: 99.85% // Head: 99.85% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (9a21a86) compared to base (f0ebc67). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1905   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files         285      285           
  Lines       40802    40813   +11     
=======================================
+ Hits        40741    40752   +11     
  Misses         61       61           

Impacted Files	Coverage Δ
src/psyclone/psyir/nodes/omp_clauses.py	100.00% <100.00%> (ø)
src/psyclone/psyir/nodes/omp_directives.py	99.80% <100.00%> (+<0.01%)	:arrow_up:
...c/psyclone/psyir/transformations/omp_loop_trans.py	100.00% <100.00%> (ø)

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[arporter]

I need to check the various examples that involve OpenMP and then this can go for review.

[arporter]

I've checked, and the various examples that show OpenMP appear to be correct.

Ready for first review from @sergisiso.

[arporter]

CI permitting, this is ready for another look. I'd prefer to leave the GOcean and LFRic defaults alone for now.