Add a current frame iterator for streamed trajectories

[PardhavMaradani]

Fixes #5183

Changes made in this Pull Request:

• Added a current frame iterator for streamed trajectories
  • The iterator is created when the frames array has a single value that is same as the current frame

This PR allows us to do something like the following:

import MDAnalysis as mda
from MDAnalysis.analysis.dssp import DSSP

top = "/tmp/imd-workshop-2025/workshop/sample_simulation/GROMACS/input/start.gro"
u = mda.Universe(top, "imd://localhost:8889")

_dssp = DSSP(u)

for i in range(5):
    ts = u.trajectory.ts
    print(ts.data, ts.frame)
    run = _dssp.run(frames=[ts.frame])
    print("".join(run.results.dssp[0]))
    u.trajectory.next()

Here is a sample output of the per-frame analysis above:

{'time': 14.76, 'dt': 0.002, 'step': 7380} 0
-E--HHHHHHHHHH-----HHHE-HHHHHHHHHHHH--E---EEE-----EEE----E------E-------------EHHHHH----HHHHHHHHHHH-----HHH-HHHHHH------HHHH-----
{'time': 14.8, 'dt': 0.002, 'step': 7400} 1
-E--HHHHHHHHHH-----HHHE-HHHHHHHHHHHH--E---EEE-----EEE----EE-----E-------------EHHHH-----HHHHHHHHHHH----HHHH-HHHHHH------HHHH-----
{'time': 14.84, 'dt': 0.002, 'step': 7420} 2
-E--HHHHHHHHHH-----HHHE-HHHHHHHHHHHH--E---EEE-----EEE----EE-----E-------------EHHHH-----HHHHHHHHHH-----HHHH-HHHHHH------HHHH-----
{'time': 14.88, 'dt': 0.002, 'step': 7440} 3
-E--HHHHHHHHHH-----HHHE-HHHHHHHHHHH---E---EEE-----EEE----EE-----E-------------EHHHH-----HHHHHHHHHHH----HHHH-HHHHHH---------------
{'time': 14.92, 'dt': 0.002, 'step': 7460} 4
-E--HHHHHHHHHH-----HHHE-HHHHHHHHHHH---E---EEE-----EEE----EE-----E-------------EHHHHH----HHHHHHHHHHHH---HHHH-HHHHHH------HHH------

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

---

📚 Documentation preview 📚: https://mdanalysis--5184.org.readthedocs.build/en/5184/

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.74%. Comparing base (9fb8b0a) to head (04f60c0).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5184      +/-   ##
===========================================
+ Coverage    92.72%   92.74%   +0.01%     
===========================================
  Files          180      180              
  Lines        22472    22487      +15     
  Branches      3188     3190       +2     
===========================================
+ Hits         20838    20856      +18     
+ Misses        1176     1174       -2     
+ Partials       458      457       -1     

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[BradyAJohnston]

This seems like a good idea to me - but I would defer to those more experienced with MDA & streaming as for why this approach maybe wasn't already taken.

[orbeckst]

I’m away from most of work (and open source) and won’t get to reviewing until the new year. But please let this not halt any discussions here!