Clarify Cantera example licenses

[ischoegl]

Summary

A recent discussion brought up the inconsistent handling of licensing language in samples (some include standard Cantera verbiage, some don't), see Cantera/cantera#1621 (comment), which states

I'm not sure what the right way to think of the examples is, in terms of licensing. I think they are frequently used in a way that is not quite consistent with what even the permissive BSD license requires, that is, with users adapting the examples to their own purposes, but not retaining the license file. At a certain level, this use case would be better accommodated by saying that the examples are public domain, but that's definitely out of scope for this PR.

I am opening an issue here to track follow-ups.

References

• Render examples using Sphinx-gallery cantera#1621