Improve fieldlist indices implementation

[sandorkertesz]

This PR refactors the implementation of the index()and indices() FieldList methods. In particular, it improves the performance when index() is called with a key not belonging to the default index keys.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.50%. Comparing base (8060550) to head (e6817eb).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #436   +/-   ##
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  Coverage    90.50%   90.50%           
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  Files          123      123           
  Lines         8274     8280    +6     
  Branches       693      693           
========================================
+ Hits          7488     7494    +6     
  Misses         655      655           
  Partials       131      131           

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