Add topoapi check + cleanup netrc check

[sssangha]

Add topoapi check + cleanup .netrc checks and move all to prep stage(s) at top of notebook.

If a given file doesn't exist, and explicit warning is passed, plus a prompt for users to enter their credentials.

[review-notebook-app]

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[bbuzzanga]

ReviewNB is not working correctly. Did you just add the block at the top for inputting netrc / opentopo credentials?

[ehavazli]

@sssangha when the notebook asks for username and password both are visible as we enter them. Is this something we may want to avoid? It isn't really that big of a deal probably but just wanted to point out.

Also when committing notebooks, it may be a good idea to "clear outputs" which would shrink the size of the notebook and make it faster to checkout.