Moving the performance.rst appendix to the POPPY docs

[mperrin]

Issue by josePhoenix
Monday Apr 06, 2015 at 18:40 GMT
Originally opened as mperrin/webbpsf#60

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Removes the discussion of calculation performance (since it really belongs in the POPPY docs) and replaces it with a link. Keeps the bulleted list of FT types for each instrument from that document, and adds it to intro.rst.

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josePhoenix included the following code: https://github.com/mperrin/webbpsf/pull/60/commits