Simplify pos_iterator and posAdc_iterator

This simplifications lead to a net HLT1 throughput improvement of +0.7% (hlt1_reco_baseline).

• Replaced convoluted bitwise operations by simple pointer arithmetic, allowing the compiler to emit more efficient asm
• Do not build iterators at construction time, to avoid paying for what we don't use
• Replace reference and pointer returns by value for CLUSTERWORD (as it is a zero cost abstraction over a simple word)

No real effort was made to further simplify the posAdc_iterator, because it is not used in the hotpath and should be deprecated and removed.

@decianm @sstahl @sponce

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• 9bdac323 - Further simplification of end iterators

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• 859aafe4 - Further simplification of end iterators

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/ci-test --merge

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assigned to @rmatev and unassigned @mstahl

Throughput tests look good! I compared
https://lhcbpr-hlt.web.cern.ch/lhcbpr-hlt/PerfTests/UpgradeThroughput/Throughput_lhcb-test-throughput2.257_Moore_hlt1_reco_ftv6_x86_64+avx2+fma-centos7-gcc9-opt_2020-01-10_02:26:27_+0100/
to
https://lhcbpr-hlt.web.cern.ch/lhcbpr-hlt/PerfTests/UpgradeThroughput/Throughput_lhcb-test-throughput.426_Moore_hlt1_reco_ftv6_x86_64+avx2+fma-centos7-gcc9-opt_2020-01-10_01:41:10_+0100/

I'm removing the label again in favor of Gaudi!1026 (we could also decide to leave it and compare tomorrow to today)

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merged

mentioned in commit fea68e5b

mentioned in merge request gaudi/Gaudi!1026 (merged)

mentioned in issue Moore#128 (closed)