refactor: make entries read-only

[timmahrt]

This PR does the following:

• renames TextgridTier.entryList to TextgridTier.entries
• makes TextgridTier.entries read-only
• bugfix for praatio_scripts.alignBoundariesAcrossTiers
• various minor refactoring and cleaning up

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (praatio_v6@b821a2a). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@              Coverage Diff              @@
##             praatio_v6      #48   +/-   ##
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  Coverage              ?   77.21%           
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  Files                 ?       21           
  Lines                 ?     2638           
  Branches              ?        0           
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  Hits                  ?     2037           
  Misses                ?      601           
  Partials              ?        0           

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