Adds distributed row gatherer

[MarcelKoch]

This PR adds a distributed row gatherer. This operator essentially provides the communication required in our matrix apply.

Besides the normal apply (which is blocking), it also provides two asynchronous calls. One version has an additional workspace parameter which is used as send buffer. This version can be called multiple times without restrictions, if different workspaces are used for each call. The other version doesn't have a workspace parameter, and instead uses an internal buffer. As a consequence, this function can only be called a second time, if the request of the previous call has been waited on. Otherwise, this function will throw.

This is the second part of splitting up #1546.

It also introduces some intermediate changes, which could be extracted out beforehand:

• a type-erased DenseCache
• makingdetail::run easier to use now part of Use index_map in distributed::matrix #1544

PR Stack:

• Add specific communicator for neighborhood communication (Again) #1780
• Add local to global index mapping #1707
• Use Index map in PGM #1639
• Adds distributed row gatherer #1589

[MarcelKoch]

One issue that I have is the constructor. It takes a collective_communicator and an index_map. The index_map already defines the communication pattern, so the collective_communicator has to match that.
One option might be to have a virtual function like

std::unique_ptr<collective_communicator> create_with_same_type(communicator, index_map);

If I can't come up with anything better, I guess I will use that.

[pratikvn]

Do we need to have the std::future setup for the release ? Can we remove that for now and just use a normal synchronous approach ? I think that is a significant change that maybe needs more thought and probably a separate PR.

[codecov]

Codecov Report

Attention: Patch coverage is 93.83754% with 22 lines in your changes missing coverage. Please review.

Please upload report for BASE (develop@2deab4b). Learn more about missing BASE report.
Report is 26 commits behind head on develop.

Files with missing lines	Patch %	Lines
core/distributed/row_gatherer.cpp	92.04%	7 Missing ⚠️
core/distributed/matrix.cpp	92.59%	4 Missing ⚠️
test/mpi/distributed/row_gatherer.cpp	96.58%	4 Missing ⚠️
core/distributed/neighborhood_communicator.cpp	0.00%	3 Missing ⚠️
include/ginkgo/core/base/segmented_array.hpp	25.00%	3 Missing ⚠️
test/mpi/distributed/matrix.cpp	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #1589   +/-   ##
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  Coverage           ?   88.64%           
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  Files              ?      849           
  Lines              ?    71101           
  Branches           ?        0           
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  Hits               ?    63029           
  Misses             ?     8072           
  Partials           ?        0           

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[sonarqubecloud]

Quality Gate passed

Issues
86 New issues
0 Accepted issues

Measures
1 Security Hotspot
95.8% Coverage on New Code
4.2% Duplication on New Code

See analysis details on SonarQube Cloud