Issue 88 and 86: Update naming to be more consistent and add new overarching AbstractModel

[seabbs]

As the title this PR closes #88 and #86.

It:

• Uses model rather than process consistently
• Renames generate_latent_process -> generate_latent to match the pattern of the other modules.
• Renames RandomWalkLatentProcess -> RandomWalk again to match the pattern from the other module.
• Renames make_epi_inference_model -> make_epi_aware again for naming consistency and as discussed (this change lacks a connected issue but I think is in scope of this PR.
• Renames epimodel to epi_model again for consistency.
• Moves all abstract types into a single file and adds a heirarchy type called AbstractModel
• Generalises scan to expect a callable of type AbstractModel

Note as this change touches a lot of files it may cause issues for #85 - sorry!

[SamuelBrand1]

This looks good.

Using DocStringExtensions lowers the pain of renaming, because done correctly it should just work