fix binary_op on operands with single element

[mtar]

Description

This PR fixes a known bug in _operations.__binary_op() where the data of an DNDarray is copied to all processors if it only contains one element.

The fix introduces the method 'Split()' to MPICommunicator for splitting it into new ones. These new ones are used for the data transfer in __binary_op()

Issue/s resolved: #866

Changes proposed:

• add MPICommunicator.Split()
• fix data transfer in __binary_op()

Type of change

• Bug fix / New feature

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 #868 (18a8297) into master (c570047) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #868   +/-   ##
=======================================
  Coverage   95.48%   95.49%           
=======================================
  Files          64       64           
  Lines        9463     9472    +9     
=======================================
+ Hits         9036     9045    +9     
  Misses        427      427           

Flag	Coverage Δ
gpu	94.62% <100.00%> (+<0.01%)	⬆️
unit	91.02% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
heat/core/_operations.py	95.87% <100.00%> (+0.10%)	⬆️
heat/core/communication.py	96.76% <100.00%> (+0.02%)	⬆️

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