Cross-platform safe temp file

[mccalluc]

• Fix /tmp might not be portable #141
• On the good side, this is more likely to work on Windows
• On the bad side, a new temp directory is created on every run
  • We could clean it up in the session.on_ended handler, but I'm not sure the complexity is worth it?

time: ~1 hour, mostly thinking about different options.

[ekraffmiller]

As another option, could we use gettempdir()? It supports different environments.
Or, can we create demo.csv just once and include it in the application?

[mccalluc]

Good call!

[ekraffmiller]

See comment about using a temp dir

[mccalluc]

It looks like that is still doesn't return something as simple as /tmp:

>>> from tempfile import gettempdir
>>> gettempdir()
'/var/folders/jm/40wztt291610f8pk5w0sw20r0000gq/T'

I'd prefer to keep the status quo: I think it's nice to have a directory that is just for this application, rather than sharing one that could inadvertently interfere with other applications.

[mccalluc]

Second, or third thought: Would be good to try checking in the generated file. One the fence, but it would mitigate this whole temp directory problem.