ENH: Restore ants.legacy interfaces

[effigies]

Summary

Fixes #3197.

List of changes proposed in this PR (pull-request)

I restored all three interfaces. I can limit it to antsIntroduction if preferred.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[satra]

we could keep all three as long as ants distributes them. if they don't i think someone would have to maintain some form of deprecation/version-based interface. now that ants is versioning, this does become a little easier.

[codecov]

Codecov Report

Merging #3222 into master will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3222      +/-   ##
==========================================
- Coverage   65.00%   65.00%   -0.01%     
==========================================
  Files         301      302       +1     
  Lines       39836    39926      +90     
  Branches     5270     5279       +9     
==========================================
+ Hits        25896    25952      +56     
- Misses      12882    12914      +32     
- Partials     1058     1060       +2     

Flag	Coverage Δ
#unittests	65.00% <66.66%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/ants/legacy.py	66.66% <66.66%> (ø)
nipype/pipeline/plugins/legacymultiproc.py	69.11% <0.00%> (-1.97%)	⬇️

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[codecov]

Codecov Report

Merging #3222 into master will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3222      +/-   ##
==========================================
- Coverage   65.00%   65.00%   -0.01%     
==========================================
  Files         301      302       +1     
  Lines       39836    39926      +90     
  Branches     5270     5279       +9     
==========================================
+ Hits        25896    25952      +56     
- Misses      12882    12914      +32     
- Partials     1058     1060       +2     

Flag	Coverage Δ
#unittests	65.00% <66.66%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/ants/legacy.py	66.66% <66.66%> (ø)
nipype/pipeline/plugins/legacymultiproc.py	69.11% <0.00%> (-1.97%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4c48a18...81e8f9e. Read the comment docs.