Writing style fix based on style guide

„Use of || is discouraged as its clarity on some output devices is low
(looking like the number 11 for instance). && is preferred over AND but
either are acceptable, and a space should always precede and follow !.”
"BASEPATH check" consistency

As described in issue #2870

+ A change in the „How to create a library” example in the user_guide
Merge pull request #2872 from darwinel/develop

Writing style fix based on style guide