Enable use of transmission techs in custom math

[brynpickering]

Adds ability to reference transmission technologies by the tech name, which will grab the relevant tech:node_to concatenated techs.

Summary of changes in this pull request:

• New expression helper function to get transmission techs, but also allow it to work in the defined where string helper function.
• New custom math to showcase it in action, creating a net import share constraint.
• Revoke option to use - in node names, replacing with _ (will need baking into a YAML schema in due course). With - there was a parsing issue raised if directly referencing the tech name in the where/expression string (- is not part of the allowed string characters).

Reviewer checklist:

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

[codecov]

Codecov Report

Merging #505 (e9fc66c) into main (b1fc928) will increase coverage by 0.05%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #505      +/-   ##
==========================================
+ Coverage   92.36%   92.42%   +0.05%     
==========================================
  Files          32       32              
  Lines        4089     4120      +31     
  Branches      964      969       +5     
==========================================
+ Hits         3777     3808      +31     
  Misses        197      197              
  Partials      115      115              

Files	Coverage Δ
src/calliope/backend/expression_parser.py	95.44% <100.00%> (+0.07%)	⬆️
src/calliope/backend/helper_functions.py	98.20% <100.00%> (+0.22%)	⬆️