Do not modify key in place within setitem, getitem

[ClaudiaComito]

Description

Issue/s resolved: #705

Changes proposed:

• both __getitem__ and __setitem__ work on copy of key if possible (i.e. if key is not a tuple)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Due Diligence

• All split configurations tested
• Multiple dtypes tested in relevant functions
• Documentation updated (if needed)
• Updated changelog.md under the title "Pending Additions"

Does this change modify the behaviour of other functions? If so, which?

no

[codecov]

Codecov Report

Merging #706 (d277e7b) into release/0.5.x (21590bd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##           release/0.5.x     #706   +/-   ##
==============================================
  Coverage          97.43%   97.43%           
==============================================
  Files                 87       87           
  Lines              17110    17112    +2     
==============================================
+ Hits               16671    16673    +2     
  Misses               439      439           

Impacted Files	Coverage Δ
heat/core/dndarray.py	96.44% <100.00%> (+<0.01%)	⬆️

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