Collapse similar codepaths, and create a mapping for single-scale simulations

[Samuel-amap]

Similarly to issue #67 , there are other places in the codebase that could benefit from being streamlined into a single codepath.

A few functions call similar-but-not-quite-the-same codepaths, which introduces extra complexity in understanding program state. Perhaps some functions could benefit from being renamed, or not overloaded, too.

ModelLists and mappings also have many similar stages, but sometimes call different functions due to the multi scale handling. It seems like it might be worthwhile to consider ModelLists as multi-scale models with only a single implicit scale (without making the API more complex on the user side). There is at least one difference worth keeping, which is that with a single scale the size of the output data can be known exactly and preallocated in advance.

[Samuel-amap]

Issues such as issue #85 might also arise less often in the future with code flow that only splits right when specialization is needed, instead of having two more distinct but still functionally similar codepaths

[VEZY]

Related to #67

[Samuel-amap]

Returning to this issue :

Given that there are some distinct differences between single and multiscale use cases, and that there is a modellist to mapping comparison that should help ensure the two cases don't desynchronise, it might be worth calling this issue closed.

See #125 and #123, and PR #122.

That being said, #67 is still a valid concern. Some codepaths might still be collapsible.

[Samuel-amap]

Marking this as closed. We're keeping both modes, for now, anyway, and #128 contains some doc example showing how to convert a single-scale to a multi-scale simulation.