Make openPMD-api default-required or advertise modified `pip install` line?

[ax3l]

@aeriforme recently got the confusing problem that openPMD-api was not automatically co-installed with a pip install openpmd-viewer.

Currently, openPMD-api is opt-in via pip install openpmd-viewer[all] or openpmd-viewer[openpmd-api].

I think we could either:

• make openpmd-api a default dependency (we support it now very widely across platforms) or
• advertise the modified pip install scripts in README & docs

What do you think? This looks like a leftover from us when we carefully adopted the new backend a few years back.