Make PixelAperture.area an abstract method

[cdeil]

@bsipocz @astrofrog Does it make sense to make PixelAperture.area an abstract method?

All tests pass before or after this change ... I made it because my editor was complaining here about the missing self.

[cdeil]

This change is not consistent with area being described as an optional method here:
http://photutils.readthedocs.org/en/latest/photutils/aperture.html#defining-your-own-custom-apertures

It says there:

area(): If convenient to calculate, this returns the area of the aperture. This speeds computation in certain situations (such as a scalar error).

But I can't find a point of use of area() ... is this implemented?

[bsipocz]

@cdeil - I think we didn't make it an abstract method as it may not be straightforward to define in future or user defined aperture classes.

However the method may well became obsolete, as it was only used for some speeding up when calculating the errors.

[larrybradley]

I think the area() method is useful. It should be easy to define for any realistic (e.g. simple geometric shape) apertures. So, I'm 👍 for making it an abstract method (with the corresponding doc changes). I also think it should be made a "lazyproperty":
http://astropy.readthedocs.org/en/stable/api/astropy.utils.misc.lazyproperty.html?highlight=lazy#astropy.utils.misc.lazyproperty

One concrete example where area() is useful is local background subtraction, where one needs the mean value within the aperture. area() wasn't used in the example here: http://photutils.readthedocs.org/en/latest/photutils/aperture.html#local-background-subtraction, but I will update that.

[cdeil]

@larrybradley So what do I have to do ... just add @lazyproperty in front of the base class method?

Feel free to do this yourself somewhere else if it's easier that explaining to me what to do.

[larrybradley]

@cdeil I've never used it with a base class. I'd try that first, but you may need to add @lazyproperty in front of every area() method too.

[cdeil]

And I guess tests should be added ... I'll try this on Saturday, but if someone gets to it before I'd be happy.