Update casjobs notebooks with login credetials once the CI can handle them

[eteq]

In #203 some notebooks were added that use MAST casjobs, which requires a specific login (more details in #203 (comment)). The notebook should require no change to get this to work, the only thing needed is the addition of CASJOBS_USERID and CASJOBS_PW envrionment variables. These, are, however "secrets" in the sense that the PW certainly should not be publicly visible. The solution is to use one of the "secrets" mechanisms backed into CI (Circle, Travis, and Github Actions all support this).

However, given the impending switch to a GA-based CI system, we decided it was better to just wait setting that up until the new CI was working. So this issue is a reminder to create those secrets (which is done via github UI but I think requires admin powers) once the GA CI is up and running.

(cc @therealzoidberg @trenton3983 )