Allow dangling ports on EVs

[mikejturner]

This PR is all above preventing the following kinds of warnings from showing when using EVs:

UserWarning: Ports {'port_ECUXTqph2bYr5BqUX4ZWWi'} are defined on nodes but are not part of an edge. This may cause erroneous optimisation results.

• Adds a new parameter allow_dangling_port to base Port model.
• EVs has 2 ports that never can connected to other ports. These now have allow_dangling_port=True set on them.
• Modify the check in verify_graph to not count any ports that have allow_dangling_port set to True.

[mikejturner]

✅ Code coverage on dangling-ports is at 81% (compared to main being at 81%)

🎉 No files have reduced coverage 🎉

Full Coverage Report

File	Details
src/echo/__init__.py	✅ 100% → 100%
src/echo/configuration.py	✅ 100% → 100%
src/echo/constants.py	✅ 100% → 100%
src/echo/exceptions.py	✅ 88% → 88%
src/echo/functional.py	✅ 100% → 100%
src/echo/models/__init__.py	✅ 100% → 100%
src/echo/models/agnostic.py	✅ 86% → 86%
src/echo/models/base.py	✅ 83% → 83%
src/echo/models/carbon.py	✅ 100% → 100%
src/echo/models/electrical.py	✅ 74% → 74%
src/echo/models/gas.py	✅ 73% → 73%
src/echo/models/prebuilt.py	✅ 56% → 56%
src/echo/models/scenario.py	✅ 100% → 100%
src/echo/models/thermal.py	✅ 81% → 81%
src/echo/objectives/__init__.py	✅ 100% → 100%
src/echo/objectives/base.py	✅ 98% → 98%
src/echo/objectives/contingency.py	✅ 93% → 93%
src/echo/objectives/power.py	✅ 86% → 86%
src/echo/objectives/tariff.py	✅ 80% → 80%
src/echo/optimiser.py	✅ 73% → 73%
src/echo/tracker.py	✅ 43% → 43%
src/echo/utils.py	✅ 61% → 61%
src/echo/validators.py	✅ 89% → 89%
src/echo/visualization.py	✅ 91% → 91%
TOTAL	✅ 81% → 81%

[mikejturner]

✅ Code coverage on dangling-ports is at 81% (compared to main being at 81%)

🎉 No files have reduced coverage 🎉

Full Coverage Report

File	Details
src/echo/__init__.py	✅ 100% → 100%
src/echo/configuration.py	✅ 100% → 100%
src/echo/constants.py	✅ 100% → 100%
src/echo/exceptions.py	✅ 88% → 88%
src/echo/functional.py	✅ 100% → 100%
src/echo/models/__init__.py	✅ 100% → 100%
src/echo/models/agnostic.py	✅ 86% → 86%
src/echo/models/base.py	✅ 83% → 83%
src/echo/models/carbon.py	✅ 100% → 100%
src/echo/models/electrical.py	✅ 74% → 74%
src/echo/models/gas.py	✅ 73% → 73%
src/echo/models/prebuilt.py	✅ 56% → 56%
src/echo/models/scenario.py	✅ 100% → 100%
src/echo/models/thermal.py	✅ 81% → 81%
src/echo/objectives/__init__.py	✅ 100% → 100%
src/echo/objectives/base.py	✅ 98% → 98%
src/echo/objectives/contingency.py	✅ 93% → 93%
src/echo/objectives/power.py	✅ 86% → 86%
src/echo/objectives/tariff.py	✅ 80% → 80%
src/echo/optimiser.py	✅ 73% → 73%
src/echo/tracker.py	✅ 43% → 43%
src/echo/utils.py	✅ 61% → 61%
src/echo/validators.py	✅ 89% → 89%
src/echo/visualization.py	✅ 91% → 91%
TOTAL	✅ 81% → 81%

[mikejturner]

✅ Code coverage on dangling-ports is at 81% (compared to main being at 81%)

🎉 No files have reduced coverage 🎉

Full Coverage Report

File	Details
src/echo/__init__.py	✅ 100% → 100%
src/echo/configuration.py	✅ 100% → 100%
src/echo/constants.py	✅ 100% → 100%
src/echo/exceptions.py	✅ 88% → 88%
src/echo/functional.py	✅ 100% → 100%
src/echo/models/__init__.py	✅ 100% → 100%
src/echo/models/agnostic.py	✅ 86% → 86%
src/echo/models/base.py	✅ 83% → 83%
src/echo/models/carbon.py	✅ 100% → 100%
src/echo/models/electrical.py	✅ 74% → 74%
src/echo/models/gas.py	✅ 73% → 73%
src/echo/models/prebuilt.py	✅ 56% → 56%
src/echo/models/scenario.py	✅ 100% → 100%
src/echo/models/thermal.py	✅ 81% → 81%
src/echo/objectives/__init__.py	✅ 100% → 100%
src/echo/objectives/base.py	✅ 98% → 98%
src/echo/objectives/contingency.py	✅ 93% → 93%
src/echo/objectives/power.py	✅ 86% → 86%
src/echo/objectives/tariff.py	✅ 80% → 80%
src/echo/optimiser.py	✅ 73% → 73%
src/echo/tracker.py	✅ 43% → 43%
src/echo/utils.py	✅ 61% → 61%
src/echo/validators.py	✅ 89% → 89%
src/echo/visualization.py	✅ 91% → 91%
TOTAL	✅ 81% → 81%