bug(medcat-trainer): CU-869ctq789 Fix saving model packs

[mart-r]

Hopefully this fixes:

https://discourse.cogstack.org/t/unable-to-save-model-from-trainer/378

The idea is to allow model pack based projects to overwrite their model packs as the CDB based ones were designed to.
Though notably without the overwrite=True option they, too, would probably have failed.

So this PR does the following:

• Creates a separate function to do the model pack overwriting if/when required
• Moves the old model pack to a temporary folder
  • Because the CAT.save_model_pack method doesn't allow (or delegate the allowing of) overwriting existing ifles
  • And if we just removed the previous file(s) I'd be afraid of leaving a state with a model pack path that refers to nothing
  • If all goes well, the copy is destroyed after
  • If something fails, attempt to move the old one back to its original location