String-based expression substitution and moar expression tests!

[mlange05]

This change separates some of the loki.expression tests into separate sub-packages and adds a new SubstituteStringExpressions utility that builds on the recently added parse_expr utility for expression substitution.

The test separation is meant to reduce the size of the general expression test, which is growing slightly out of proportions. I've also added some explicit test of the much used SubstituteExpressions transformer.

Steps in detail:

• A new loki.expression.test.test_expr_visitors sub-package with some existing tests and new tests for expression substitution; this includes an added test for FindVariables which was previously only covered implicitly (it seems)
• Separating out the tests of for the new expression parser into loki.expression.tests.test_parser
• New SubstituteStringExpressions transformer that works pretty much as the SubstituteExpressions but uses parse_expr to use string mapping as input.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/366/index.html

[codecov]

Codecov Report

Attention: Patch coverage is 99.80952% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.42%. Comparing base (fce5733) to head (ad4b4d8).
Report is 54 commits behind head on main.

Files with missing lines	Patch %	Lines
loki/expression/tests/test_parser.py	99.77%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #366   +/-   ##
=======================================
  Coverage   95.41%   95.42%           
=======================================
  Files         178      180    +2     
  Lines       37312    37387   +75     
=======================================
+ Hits        35600    35675   +75     
  Misses       1712     1712           

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	95.40% <99.80%> (+<0.01%)	⬆️

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[reuterbal]

The SubstituteStringExpressions utility is a really nice one! Obviously, loads of ways how someone can use this in dangerous ways, so we need to watch out for this.

The test clean-up and extended explicit coverage is much appreciated!

Good to go from me!

[reuterbal]

I've not gone through this in detail but I'm trusting this is largely just a source move.

[mlange05]

Yes, it is. I tried to be careful here. 😉

[reuterbal]

Suggested change

	use external_mod, only: rick
	use external_mod, only: rick
	implicit none

missing also from a few other places in this file, 'cause, wouldn't want to risk Rick letting me down due to implicit typing after all.

[mlange05]

Rick would never tell a lie and hurt you. Fixed now 😉