Update .gitignore list

[geographika]

Fixes #250
Also ignores changed output generated in #228 and 10-populated-places-ba.json (which is not in the repo but generated when working through the notebook).

[justb4]

Be sure to coordinate with @pvgenuchten for the COG file:

• check the name
• or better: I suggest to create an output dir under notebooks and place a .gitkeep file there and maybe even write all generated output there (iso in test) then with one "catchall" .gitignore rule, . See data/output, similar.

[geographika]

This PR is just to update the current approach. Writing all outputs to a single directory makes sense, but some Notebook outputs are checked-in to the repo, others are in different locations, so these would all need to be reviewed. Probably not something to do within hours of freezing development.

@pvgenuchten - please check that the COG file generated in #248 matches the name here (or feel free to update with a new location).

[pvgenuchten]

agree to prevent file creation in other then ../data/output
however in some cases these files need to be hosted, which is currently not possible in ../data
i'm moving away from the cog in openlayers sample, it came too early in the workshop