Bug/591 slicing memory issues

[coquelin77]

Description

getitem/setitem now use tuples for most key types.
Advanced indexing is also implemented via this approach. keys for advanced indexing can be lists, torch.Tensors, or ht.DNDarrays. This means that the results of where and nonzero can be fed directly into another DNDarray as a key. However, it also means that lists are treated differently than tuples when given as a key

Issue/s resolved: #591 #505

Changes proposed:

• most keys in getitem/setitem create a tuple for a given key
• removal of other types of key options in getitem/setitem
• reduce_op neutral value array created using the gshape instead of the lshape
• addition of advanced indexing

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

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?

yes, any function which uses where or nonzero can now use the DNDarray instead of a list.

[codecov]

Codecov Report

Merging #594 into master will increase coverage by 0.01%.
The diff coverage is 97.72%.

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
+ Coverage   96.47%   96.48%   +0.01%     
==========================================
  Files          75       75              
  Lines       15169    15214      +45     
==========================================
+ Hits        14634    14679      +45     
  Misses        535      535              

Impacted Files	Coverage Δ
heat/core/dndarray.py	96.90% <97.09%> (+0.05%)	⬆️
heat/core/arithmetics.py	99.19% <100.00%> (ø)
heat/core/indexing.py	100.00% <100.00%> (ø)
heat/core/manipulations.py	99.51% <100.00%> (ø)
heat/core/operations.py	92.81% <100.00%> (+0.03%)	⬆️
heat/core/statistics.py	97.82% <100.00%> (ø)
heat/core/stride_tricks.py	100.00% <100.00%> (ø)
heat/core/tests/test_arithmetics.py	97.88% <100.00%> (+<0.01%)	⬆️
heat/core/tests/test_dndarray.py	99.37% <100.00%> (+<0.01%)	⬆️
heat/core/tests/test_indexing.py	92.75% <100.00%> (ø)
... and 2 more

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[Markus-Goetz]

Mostly minor changes. A job well done. Thanks Daniel