[SYSTEMDS-3800] Add tests to increase code coverage of FED instructions

[gaturchenko]

The tests for the following federated instructions have been added:

Central moment (weighted case)
Ternary aggregation (cumulative product & sumproduct)
Ternary instruction (account for non-literal scalar inputs)
The TernaryFEDInstruction was also slightly modified, since in its previous implementation the processMatrixScalarInput method returned an "unknown variable" error when processing an ifelse instruction with one matrix input and non-literal scalar inputs.

[codecov]

Codecov Report

Attention: Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.

Project coverage is 71.84%. Comparing base (58490d3) to head (54a12dc).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...untime/instructions/fed/TernaryFEDInstruction.java	82.60%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2148      +/-   ##
============================================
+ Coverage     71.77%   71.84%   +0.07%     
- Complexity    43727    43757      +30     
============================================
  Files          1441     1441              
  Lines        165952   165971      +19     
  Branches      32383    32388       +5     
============================================
+ Hits         119104   119242     +138     
+ Misses        37572    37453     -119     
  Partials       9276     9276              

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[mboehm7]

LGTM - thanks for the additional tests and improved code coverage @gaturchenko. A minor thing for the future: please do not include the commits from previous PRs which creates lots of merge conflicts during the rebase/squash process. Instead I just picked the actual 3 new commits. Thanks.