Merge branch 'feature/track-deleted-entities' into 'master'

Track number of deleted entities and add n_logical_*() methods as proposed in #8

See merge request !38
12 jobs for master in 5 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #58596
VS2013
VS2013-32-bit

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passed #58595
VS2013
VS2013-64-bit

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VS2015
VS2015-32-bit

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VS2015
VS2015-64-bit

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passed #58598
VS2017
VS2017-32-bit

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passed #58591
VS2015
VS2017-64-bit

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passed #58588
Linux
clang-c++11

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cppcheck

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Linux
doc

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Linux
gcc-c++11

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Apple
macos-c++11

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failed #58592
VS2017
VS2017-32-bit

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