Generic algorithms + example/test for writing a barrier with ZipSelections

Components + a draft for a barrier with the ZipSelection framework. So far (at least the last time I checked) the Barrier only supports KeyedContainers. The concept of a barrier is explained here

port from previous works on Pr::Selection.

cc @nnolte

• open for input if Barriers (with SOAExtensions) is actually something we want/need in the scope of barriercontrolflow

wrt. previous barrier implementations, a scatterer is not needed:

• A transformer with Zipping::transform would just consume the original "all objects" container and put out a new container that is zip compatible. Scattering doesn't need to happen as the ExportedSelection that goes into the barrier also serve as "readers" (constructor arguments for SelectionView) of the output of the transformer.

This is a reopening of !1914 (closed)

added Core new feature labels

mentioned in merge request !1914 (closed)

mentioned in issue Moore#163 (closed)

mentioned in issue Moore#168 (closed)

mentioned in issue Moore#170 (closed)

assigned to @frodrigu

/ci-test --merge

• [2020-05-28 17:25] Validation started with lhcb-master-mr#865
• [2020-05-30 00:06] Validation started with lhcb-sanitizers#578
• [2020-05-30 00:08] Validation started with lhcb-lcg-dev4#1296
• [2020-05-30 00:08] Validation started with lhcb-head#2604
• [2020-05-30 00:09] Validation started with lhcb-gaussino#730
• [2020-05-30 00:09] Validation started with lhcb-test-throughput2#356
• [2020-05-30 00:09] Validation started with lhcb-gaudi-head-py3#370
• [2020-05-30 00:09] Validation started with lhcb-lcg-test#49
• [2020-05-30 00:09] Validation started with lhcb-gauss-dev#2403
• [2020-05-30 00:13] Validation started with lhcb-gaudi-head#2616
• [2020-05-30 00:16] Validation started with lhcb-dd4hep#535
• [2020-05-31 00:05] Validation started with lhcb-lcg-dev3#1284
• [2020-05-31 00:06] Validation started with lhcb-test-throughput2#357
• [2020-05-31 00:06] Validation started with lhcb-lcg-test#50
• [2020-05-31 00:08] Validation started with lhcb-lcg-dev4#1297
• [2020-05-31 00:09] Validation started with lhcb-dd4hep#536
• [2020-05-31 00:09] Validation started with lhcb-gaudi-head-py3#371
• [2020-05-31 00:09] Validation started with lhcb-gaudi-head#2617
• [2020-05-31 00:09] Validation started with lhcb-gauss-dev#2404
• [2020-05-31 00:09] Validation started with lhcb-test-efficiency#88
• [2020-05-31 00:09] Validation started with lhcb-head#2605
• [2020-05-31 00:10] Validation started with lhcb-sanitizers#579
• [2020-05-31 00:25] Validation started with lhcb-coverity#509
• [2020-06-01 00:06] Validation started with lhcb-gauss-dev#2405
• [2020-06-01 00:06] Validation started with lhcb-lcg-dev3#1285
• [2020-06-01 00:09] Validation started with lhcb-lcg-dev4#1298
• [2020-06-01 00:13] Validation started with lhcb-head#2606
• [2020-06-02 00:05] Validation started with lhcb-gaussino#732
• [2020-06-02 00:06] Validation started with lhcb-gauss-dev#2406
• [2020-06-02 00:06] Validation started with lhcb-dd4hep#537
• [2020-06-02 00:08] Validation started with lhcb-lcg-dev4#1299
• [2020-06-02 00:08] Validation started with lhcb-lcg-dev3#1286
• [2020-06-02 00:08] Validation started with lhcb-sanitizers#580
• [2020-06-02 00:09] Validation started with lhcb-gaudi-head-py3#372
• [2020-06-02 00:09] Validation started with lhcb-lcg-test#51
• [2020-06-02 00:10] Validation started with lhcb-test-throughput2#358
• [2020-06-02 00:11] Validation started with lhcb-test-efficiency#89
• [2020-06-02 00:14] Validation started with lhcb-gaudi-head#2618
• [2020-06-02 00:19] Validation started with lhcb-head#2607

resolved all threads

assigned to @jonrob and unassigned @frodrigu

approved this merge request

Looks OK, but just going to double check this one in the nightlies over the w/e.

added all-slots label

removed all-slots label

@jonrob The nightly appears to be fine.

I'm missing a bit the context here. Is this MR needed for something, or just a cleanup by Paul, or ?? @apearce is this something you need on the selections side ?

resolved all threads

merged

mentioned in commit 52dedcf7