IR: Symbol management on scoped nodes

[mlange05]

Note: This PR sits on top of #372, so I'm only filing a draft before anticipated rebase.

This PR brings a set of convenience methods and expression constructors to the ScopedNode and ProgramUnit classes. The idea is that eventually, Subroutine and Module classes should become scoped nodes and the expression management API should be equivalent between them.

The idea is to add the following three utilities to the scope objects themselves, to enable quick(er) expression generation:

• .get_symbol(name) - utility to look up a symbol by it's name. This will first find it's true owning scope and attach the symbol.
• .Variable(name, ....) - a shortcut to the general Variable constructor that will attempt to build array and scalar symbols, depending on available type information
• .parse_expr - a shortcut to the general expression parser that can generate nested expression trees from pure strings.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/375/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.50%. Comparing base (cc179b3) to head (ee2751c).
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #375      +/-   ##
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+ Coverage   95.48%   95.50%   +0.01%     
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  Files         185      186       +1     
  Lines       38646    38766     +120     
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+ Hits        36902    37022     +120     
  Misses       1744     1744              

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	95.48% <100.00%> (+0.01%)	⬆️

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[reuterbal]

Many thanks, this is fantastic!

(And I'm already dreading the follow-on PR that applies the strict Scope-local construction of symbols throughout... ;-)