Port over loop handling utils from PSyACC

[jwallwork23]

Partially addresses #2717.

This PR merges functions from the loop module of PSyACC into PSyclone:

• is_outermost checks that the Loop doesn't have any ancestors which are Loops.
• is_perfectly_nested checks that the Loop nest starting at a given Loop is "loops all the way down" until the last.
• is_simple checks that a Loop nest is perfect and that the deepest level only contains literal assignments (i.e., the loop nest is trivially parallelisable).
• nest_variable_names extracts the tuple of variable names for the Loop nest starting at a given Loop.

[sergisiso]

@jwallwork23 Isn't this equivalent to the Loop.independent_iterations method? (I think we considered this name for that method but decided the independent iterations could be more generic)

[jwallwork23]

@jwallwork23 Isn't this equivalent to the Loop.independent_iterations method? (I think we considered this name for that method but decided the independent iterations could be more generic)

Oh good, if there is already an equivalent method then that's one less thing to merge in :) Thanks!

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.86%. Comparing base (8367de1) to head (32dba72). Report is 231 commits behind head on master.

Files with missing lines	Patch %	Lines
src/psyclone/psyir/nodes/loop.py	96.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2754 +/- ## ========================================== - Coverage 99.86% 99.86% -0.01% ========================================== Files 354 354 Lines 49010 49041 +31 ========================================== + Hits 48946 48976 +30 - Misses 64 65 +1 ```

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