(Towards #1823) Permit only module-inlined Kernels to be transformed

[arporter]

No description provided.

[arporter]

Possibly I need to store the RoutineSymbol associated with a Kernel within the Kernel class (c.f. Call.reference). That way, I can tell whether it has been module-inlined or not and can get rid of the special flag for that.

[arporter]

Note to self: I will need to remove the kernel_outputdir test fixture.

[codecov]

Codecov Report

❌ Patch coverage is 97.52066% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.88%. Comparing base (ad770b6) to head (b269f7a).

Files with missing lines	Patch %	Lines
...mon/transformations/kernel_transformation_mixin.py	90.47%	2 Missing ⚠️
src/psyclone/psyGen.py	95.00%	1 Missing ⚠️

❌ Your project check has failed because you have indirect coverage changes. Learn more about Unexpected Coverage Changes and reasons for indirect coverage changes.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3294      +/-   ##
==========================================
- Coverage   99.95%   99.88%   -0.08%     
==========================================
  Files         380      381       +1     
  Lines       53922    53847      -75     
==========================================
- Hits        53900    53786     -114     
- Misses         22       61      +39     

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