Decision vars & global exprs in where strings

[brynpickering]

Fixes issue(s) #450

Summary of changes in this pull request:

Since there is a strict order of things in the backend (parameters -> variables -> expressions -> constraints -> objective), we will have already constructed variables by the time we construct expressions and will have access to both variables and expressions by the time we reach constraints. Therefore, their existence can be checked in the constraint/objective where string. It makes some things less verbose.

Reviewer checklist:

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

Merging #490 (94a64e1) into main (cbed4c8) will decrease coverage by 2.79%.
Report is 4 commits behind head on main.
The diff coverage is 95.81%.

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
- Coverage   94.98%   92.20%   -2.79%     
==========================================
  Files          31       32       +1     
  Lines        3890     4015     +125     
  Branches        0      961     +961     
==========================================
+ Hits         3695     3702       +7     
- Misses        195      196       +1     
- Partials        0      117     +117     

Files	Coverage Δ
src/calliope/__init__.py	100.00% <ø> (ø)
src/calliope/_version.py	100.00% <ø> (ø)
src/calliope/backend/helper_functions.py	99.35% <ø> (ø)
src/calliope/backend/where_parser.py	98.55% <100.00%> (ø)
src/calliope/cli.py	81.67% <100.00%> (ø)
src/calliope/core/__init__.py	100.00% <ø> (ø)
src/calliope/core/attrdict.py	99.50% <ø> (ø)
src/calliope/core/io.py	91.75% <ø> (ø)
src/calliope/core/model.py	93.23% <100.00%> (ø)
src/calliope/core/util/generate_runs.py	87.09% <ø> (ø)
... and 22 more

[sjpfenninger]

Good to go, with the proviso that the docs should highlight how vars and global_expressions are allowed in where strings and maybe some tips on how to differentiate between them when reading existing constraints