exists -> active

[brynpickering]

Fixes last part of issue #448

Summary of changes in this pull request:

• exists -> active across the board

Reviewer checklist:

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

[codecov]

Codecov Report

Merging #500 (f0a05ad) into main (cbed4c8) will decrease coverage by 2.79%.
Report is 11 commits behind head on main.
The diff coverage is 95.69%.

@@            Coverage Diff             @@
##             main     #500      +/-   ##
==========================================
- Coverage   94.98%   92.20%   -2.79%     
==========================================
  Files          31       32       +1     
  Lines        3890     4016     +126     
  Branches        0      957     +957     
==========================================
+ Hits         3695     3703       +8     
- Misses        195      198       +3     
- Partials        0      115     +115     

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% <100.00%> (ø)
src/calliope/core/model.py	94.05% <100.00%> (ø)
src/calliope/core/util/generate_runs.py	87.09% <ø> (ø)
... and 22 more