Description / tutorial for setting up BCs

[lcarasik]

The goal is to answer this question for the user:

“If using external mesh or ones without passive scalars/temperature, do this”?

This will supplement this page:

http://nek5000.github.io/NekDoc/problem_setup/boundary_conditions.html

[smharper]

I'd like to reinforce this issue a bit. I think some further docs on this topic would be helpful for the velocity fields as well as the passive scalars.

In particular, I would like for the docs to describe the bc and cbc arrays, and how to go about reading/modifying them in usrdat2. Essentially, this information from the users-group. Relatedly, maybe we should also talk about how to interrogate these arrays in userbc in order to e.g. handle multiple inlets with different velocity BCs.

[lcarasik]

@smharper I appreciate your encouragement! I agree with you completely on this approach.

I agree we need to address BCs and CBC array + how you create your mesh impacts both + multiple inlets is something I've encountered before and I might be able to throw an example together.