ENH: Slightly simplify frame_indices array construction

[effigies]

Turning #1387 (comment) into a PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.34%. Comparing base (f81f5a7) to head (df638fd).
Report is 31 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1404      +/-   ##
==========================================
- Coverage   95.34%   95.34%   -0.01%     
==========================================
  Files         208      208              
  Lines       29774    29772       -2     
  Branches     3356     3355       -1     
==========================================
- Hits        28387    28385       -2     
  Misses        948      948              
  Partials      439      439              

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[effigies]

My initial proposal turned out to be wrong, as atleast_2d adds dimensions to the front instead of the back. This instead maps atleast_1d over the arrays that are scalar or 1d, which feels slightly clearer in intent to me, but not so much that I'd insist on this.

Closing as not worth the churn.