Methods and configuration to match and have default `splits`

[butler54]

Issue description / feature objectives

Looking a methods such as ssp-assemble, they will leverage load-distributed, however, they will only write one coherent document which may not make sense to the end user.

What a user would like is either:

1. Split to a configurable standard
2. split to what currently exists in the directory that is being read.

That implies a few pieces of functionality:

1. A function which given a model directory, discovers what the 'split element path' would be. e.g. given

ssp.json
ssp/metadata.json

the split element path would be ssp.metadata.

2. a method given a named model location, an in-memory object representation, and the split element path write out the split directory tree. Note technically this is available from the repository api, however, refactoring may be useful

3. Presuming we do not have an existing element-string lookup, from the config file, the split-element-path and apply (2) rather than writing out the whole file by default.

Caveats / Assumptions

Completion Criteria