Merge branch 'include-fix' into 'master'

Use relative include path, fixes #15

Closes #15

See merge request !72
9 jobs for master in 4 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #79740
VS2015
VS2015-32-bit

00:01:13

passed #79739
VS2015
VS2015-64-bit

00:01:26

passed #79738
VS2017
VS2017-32-bit

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passed #79737
VS2015
VS2017-64-bit

00:01:23

passed #79734
Linux
clang-c++11

00:04:41

passed #79736
Linux
cppcheck

00:03:46

passed #79741
Linux
doc

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passed #79733
Linux
gcc-c++11

00:04:52

passed #79735
Apple
macos-c++11

00:01:51