Add high-level docs page about flux and surface brightness about reproject HOT 7 CLOSED

@cdeil - actually both should be preserving surface brightness (well, the interpolation only roughly), but if the spherical_intersect is not then it's a bug :) The default for Montage is to preserve surface brightness. I agree this should clearly be documented.

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@ellisowen You looked at this ... can you make a pull request that adds a paragraph to the docs and a few unit tests.

I think using a flat image as input of value 1 (say CAR projection with 5 x 5 pixels) and reprojecting it to factor 2 higher and lower resolution and checking that the output image in both cases has again value 1 would be a nice demonstration proving that both routines reproject preserve "surface brighness", not "flux per pixel".

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@ellisowen @cdeil - just to check, have you had a chance to work on this?

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Sorry, I haven't had chance yet (although I should have a bit of time to do this later this week).

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@ellisowen - just to check, do you still plan to look into this?

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Should be addressed by #47 (which adds numerical tests) and #48 (which adds a paragraph to the docs)

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(specifically, the test is f13fd9b)

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