BF+TST: Fix 'frame_order' for single frame files

[moloney]

The _frame_indices attribute on the MultiframeWrapper was empty for single frame files, causing use of the frame_order property to raise an exception.

Also, don't assume StackID is int (it is a str), don't assume DimensionIndexValues has more than one value. Fix how we assign multiple values to DimensionIndexValues when making fake test data (need to pass list instead of tuple).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.38%. Comparing base (d9c479a) to head (b1eb9b0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1387   +/-   ##
=======================================
  Coverage   95.37%   95.38%           
=======================================
  Files         207      207           
  Lines       29672    29697   +25     
  Branches     3018     3020    +2     
=======================================
+ Hits        28300    28325   +25     
  Misses        934      934           
  Partials      438      438           

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