Test error message when accessor is not open. Also an additional test of writeconst vs. read/modify/write.

Devops and documentation

See merge request !60

Add warning about OpenZGY/Python not being actively maintained. Use the Python wrapper around OpenZGY/C++ if possible.

Some performance work

See merge request !59

Add --omp-nest option to the copy application. May be needed when testing different threading models.

Sync with the internal SLB repository. More devops.

See merge request !58

Several unrelated minor changes.

See merge request !57

Remove OpenZGY builds for Ubuntu Xenial and Debian Stretch. The alternative would be to upgrade to a new python (as Python 3.6 is end of life) but I doubt anybody is using those distros any longer.

More multi-threading

See merge request !56