Fix DataFrame.select when no set index

[PProfizi]

Requires ansys/pydpf-core#1473

[wiz-inc-572fc38784]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.12%. Comparing base (11ffa75) to head (ee444fc).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #613   +/-   ##
=======================================
  Coverage   85.11%   85.12%           
=======================================
  Files          52       52           
  Lines        5173     5175    +2     
=======================================
+ Hits         4403     4405    +2     
  Misses        770      770           

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