Merging models at runtime

[plonk2327]

It would be useful to be able to select two models within the UI and merge them in memory at runtime, specifying a ratio between them, in a similar way to scripts such as this one: https://github.com/eyriewow/merge-models

This could be used to generate comparisons of merges at different ratios using the X/Y plot script, without having to generate lots of .ckpt files on the disk.

It would then be a useful option to save a merged model to the disk for continued use, once you have found a ratio that you are happy with.