Feature request: custom names for model output

[mhovd]

Requested by Jasper

Currently, runs are stored in numeric, incrementally named folders, e.g. 1, 2, etc.
Instead, it would be more useful to be able to name the folder anything, e.g. two_comp.

[mhovd]

An alternative solution would be to not store the results in a folder at all, but a single, joint file containing all the outputs.
Obviously this would be a breaking change, but we could possible have a flag in the call to fit, e.g. result = "mymodel.pmx", which would save the results in that collated file.

The underlying representation could be anything, e.g. zip or JSON.