Transition and measurement models for road networks

[sglvladi]

This PR aims to add the following:

• RoadNetwork type:
  • A subclass of networkx.DiGraph, with some limitations on the allowed node and edge attributes, and some additional methods.
• OptimalPathToDestinationTransitionModel transition model:
  • A transition model that models a target travelling along the optimal path to a destination, on a given road network.
• OptimalPathToDestinationMeasurementModel measurement model:
  • A measurement model that projects the target's position on the road network to a 2D position.

TODO

• [x] Add tests
• [ ] Remove/Reformat example

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.17647% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 93.56%. Comparing base (7fce938) to head (0f13262). Report is 23 commits behind head on main.

Files	Patch %	Lines
stonesoup/models/transition/graph.py	90.10%	4 Missing and 5 partials :warning:
stonesoup/models/measurement/graph.py	92.68%	0 Missing and 3 partials :warning:
stonesoup/functions/graph.py	98.03%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #812 +/- ## ========================================== + Coverage 93.49% 93.56% +0.06% ========================================== Files 200 204 +4 Lines 12715 13055 +340 Branches 2615 2685 +70 ========================================== + Hits 11888 12215 +327 - Misses 585 590 +5 - Partials 242 250 +8 ``` | [Flag](https://app.codecov.io/gh/dstl/Stone-Soup/pull/812/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/dstl/Stone-Soup/pull/812/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl) | `65.26% <22.05%> (-1.16%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/dstl/Stone-Soup/pull/812/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl) | `89.26% <96.17%> (+0.22%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl#carryforward-flags-in-the-pull-request-comment) to find out more.

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[sglvladi]

@jswright-dstl, @sdhiscocks this PR is ready for review. I've left the example in for now to help with the review, but plan to remove it before merging.