refactor fitting routines

[bernstei]

move ace_fit.jl into ACE1pack.

move gap_multistage outside of workflow, probably in QUIP

• move checking for completed work into separate function, to be called explicitly from workflow
• move actual fitting work into separate function
• get rid of input/output file massaging, restart is broken anyway. Just default to output_files=rundir
• get rid of GAP-specific env var setting based on EXPYRE vars, people find it unexpected - make user do it manually in remote_info
• remaining code is essentially independent of fit() - turn into decorator?
• move descriptor_heuristics.py into QUIP as well

same for gap_simple.py, maybe also ace.py

[bernstei]

ace_fit.jl has been moved