Flag peaks2maps for removal in 0.0.13/0.1.0

[tsalo]

Closes #615, though I won't mark this as ready to review this until we have a consensus in that issue.

Changes proposed in this pull request:

• Add "deprecated" directive to docstrings of functions and classes related to the Peaks2Maps algorithm.
• Add a deprecation warning to Peaks2MapsKernel.

[codecov]

Codecov Report

Merging #616 (1669c5c) into main (16f06b0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #616   +/-   ##
=======================================
  Coverage   85.76%   85.76%           
=======================================
  Files          40       40           
  Lines        4390     4391    +1     
=======================================
+ Hits         3765     3766    +1     
  Misses        625      625           

Impacted Files	Coverage Δ
nimare/cli.py	97.26% <ø> (ø)
nimare/extract/extract.py	51.48% <ø> (ø)
nimare/meta/utils.py	88.70% <ø> (ø)
nimare/workflows/peaks2maps.py	37.50% <ø> (ø)
nimare/meta/kernel.py	96.51% <100.00%> (+0.02%)	⬆️

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[tsalo]

No objections have been raised, so I'm merging this.