Small change to gunzip to allow better restarting

[jmmshn]

Small change to gunzip to allow better restarting

Proposed change to make resuming of long-running calculations easier

If we want to resume a calculation
VaspJob in custodian has an auto_continue argument. This makes sure that CONTCAR is copied to POSCAR whenever possible.
https://github.com/materialsproject/custodian/blame/26bf28a4b15b1253c955f3f8d6bd427bbc398e3a/custodian/vasp/jobs.py#L209-L229

So setting something like this at job creation or setting it in a restart script/CLI will be useful.
For a given maker:

BaseVaspMaker.run_vasp_kwargs = {"vasp_job_kwargs" : {"auto_continue" : True}}

However, most of the VASP jobs in atomate2 will copy from prev_dir first and thus overwrite the current state of the directory. In most cases, this will just error out because force_overwrite=False by default.
For proper restarting: the copy_vasp_files function should make sure it does not overwrite existing files.
This is currently not possible. This PR changes the gunzip function to accept force="skip" as an argument.

[codecov]

Codecov Report

Merging #476 (9f5c41f) into main (18d014a) will decrease coverage by 0.19%.
The diff coverage is 27.77%.

❗ Current head 9f5c41f differs from pull request most recent head c8eb162. Consider uploading reports for the commit c8eb162 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #476      +/-   ##
==========================================
- Coverage   65.71%   65.52%   -0.19%     
==========================================
  Files          78       78              
  Lines        7462     7476      +14     
  Branches      964      970       +6     
==========================================
- Hits         4904     4899       -5     
- Misses       2255     2275      +20     
+ Partials      303      302       -1     

Files Changed	Coverage Δ
src/atomate2/vasp/files.py	87.71% <ø> (ø)
src/atomate2/utils/file_client.py	43.22% <27.77%> (-0.60%)	⬇️

... and 2 files with indirect coverage changes

[utf]

I just made some edits to add the same feature to gzip files as well, to make sure the APIs are consistent. Would you be able to add a corresponding test for the force mode for gzipping files?

I also butchered the linting, sorry about that.

[utf]

Actually, nvm. This looks good!