Fix accumulate_metadata replacing coordinates with Nones

[gjoseph92]

Fixes #78

While reviewing #87, I realized there were many things wrong with accumulate_metadata. The function just had too many cases and was trying to do too many things. #40 allowed for some incorrect code paths which we aren't actually using, but would be annoying to handle correctly. So instead, I basically redid #40, splitting accumulate_metadata into one function for the normal properties case, and one for the asset-metadata case where we only care about fields that have the same value everywhere.

@scottyhq I believe this fixes your example, but would be good to confirm:

[scottyhq]

@scottyhq I believe this fixes your example, but would be good to confirm:

Looks good to me, thanks for working on it! Out of curiosity how are you getting the times output in each cell in the screenshot you included? Feel free to close #87 in favor of this.

[gjoseph92]

how are you getting the times output in each cell in the screenshot you included?

Ah, this is just using the jupyter notebook feature in VSCode: https://code.visualstudio.com/docs/datascience/jupyter-notebooks. It's gotten a lot better in the last year, you can pretty much use it for anything that doesn't require widgets (and then you get the much better editor and autocomplete).