Transform

[crawfordsm]

closes #67 and closes #66. Adds transformations of the CCDData object although uncertainties may not be handled correctly. It also handles rebinning of ccddata objects.

[mwcraig]

By all planes you mean image, mask and uncertainty, right?

[crawfordsm]

fixed

[mwcraig]

Several specific comments are above -- this seems like a solid start.

A few broad questions:

• Should rebin be doing some kind of interpolation?
• [May be broader than this PR, really] Should we even try to transform the uncertainties and masks? With the uncertainties in particular, we are not changing the class of the uncertainty even though we are changing the uncertainty array to something that may no longer be of that class. I think it would be safer to raise an error if the data has an uncertainty.
• Should units also be transformed? If so, it can be done pretty easily by calling the transform function with a Quantity like 1 * ccd.unit (almost all numpy functions work well with Quantity but not all work with a bare Unit).
• Could you add mentions of these to the appropriate places in the docs?

[crawfordsm]

As for transforming uncertainties and masks, there are a small set of cases where they will be handled fine (integer shifts) and then a much larger set of cases where they will have problems (just about anything else). Masks I think are handled correctly, but with the statement that anything that once had a bad pixel in it, now is a bad pixel. That is at least safe even though a more complex algorithm may still be able to handle and still include those pixels.

However, I don't think there is anything in existence right now that would handle transforming uncertainty frames correctly and the correlation between the pixels. This will probably be close for most cases, but I think this would be an issue to be improved upon in future versions.

[crawfordsm]

As for Quantity, these should only be transformations in terms of geometric transforms. I think the user should do something else if they want to do transformations of the units. I've added a note as such.

[mwcraig]

It is getting increasingly tempting to not deal with uncertainties at all :) -- it will be useful to hear from users what they want/need.

I thought it was likely the case that transforms should be purely geometrical -- sounds good!