Improved TokamakSource (no properties)

[LiamPattinson]

This is an alternate take on Pull Request https://github.com/fusion-energy/openmc-plasma-source/pull/48 in which properties are avoided. Both versions behave essentially the same. I'd argue this version is simpler while the other is more robust. I also think the property version leads to a cleaner interface, but may be harder to understand and maintain.

Pros of property version:

• Can't modify attributes to something inappropriate after initialisation
• Cleaner TokamakSource.__init__, and constraints are clearly defined above.

Cons of property version:

• Property factories are confusing, so it might be difficult to work with later.
• Clutters up TokamakSource.__dict__ and dir(TokamakSource), but this is unlikely to matter for most users.

Let me know which version appeals most, and whether you'd like me to change anything I've added.

[RemDelaporteMathurin]

Hi @LiamPattinson I like the approach of the other PR. If in time we realise it's confusing, it will always be possible to go back to classic checking in the init.