Adding materials to DEMO

[oliverfunk]

This PR setups the config and mechanics for specifying materials for each DEMO PhysicalCompoents in XYZ.

Part of the challenge in doing this is:

• Getting the necessary and correct material definitions data (in materials.json, mixtures.json)
• Specifying which components get what material

For some components, the geometry may need to be altered to define different parts of the component.
The breeding blanket for example does not have regions defined for the breeding zone, manifold etc.

Linked Issues

Closes #{ID}

Description

Interface Changes

Checklist

I confirm that I have completed the following checks:

• Tests run locally and pass pytest tests --reactor
• Code quality checks run locally and pass pre-commit run --from-ref develop --to-ref HEAD
• Documentation built locally and checked sphinx-build -W documentation/source documentation/build

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 10 lines in your changes missing coverage. Please review.

Project coverage is 76.49%. Comparing base (25793b8) to head (982e9be).

Files with missing lines	Patch %	Lines
bluemira/materials/cache.py	50.00%	10 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3680   +/-   ##
========================================
  Coverage    76.48%   76.49%           
========================================
  Files          230      230           
  Lines        26767    26791   +24     
========================================
+ Hits         20474    20493   +19     
- Misses        6293     6298    +5     

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[je-cook]

On the whole its a really nice start, I like how you load and create the material cache.
There are probably some rough edges we can smooth and everything I mentioned is minor. I'll have a go with it and come back round again

[je-cook]

Thoughts welcome:
Possibly build_config['material'] should be autocreated in the reactor config and then the materials entry could be a default dict https://docs.python.org/3/library/collections.html#collections.defaultdict. It might mean we can chain gets. Also could well have unintended side effects...probably a bad idea.

[je-cook]

The component material maybe should go as a class var so it can be accessed outside? Same as we do in some places for the PhysicalComponent name (and you've done below).

[je-cook]

maybe also we could have a wrapper function that does some of this so the user only passes in a dictionary and a key, possibly harder to work out whats going on:

get_cached_material_for_component(self.build_config, self.CASING)

[je-cook]

I really like these changes, nice!

[je-cook]

Note to change these back before merge

[je-cook]

And here, change back before merge, I'm not going to comment them all

[je-cook]

Could these paths come directly from the reactor config?