pynwb not available for powerlinux on conda-forge

[dfsp-spirit]

Describe the bug
Our internal Gitlab tests fail with latest dev version because of the new dependency pynwb, which is not available for powerlinux on condaforge.

To Reproduce
Check gitlab runner log

Expected behavior
Syncopy runs on powerlinux

Suggestion
Turn pynwb into an optional dependency, skip pynwb tests if not available.

[dfsp-spirit]

For now, we install pynwb via pip on the CI system, so that our pynwb-related tests run. Conda package for pynwb is in the making, see pynwb issue 1714.