zernike.arbitrary_basis still sometimes clips the edges of apertures

[kvangorkom]

I ran across this when looking at the basis calculated for a square aperture:

This appears to be the result of using the wrong formula for the center of the array when calculating the padding for the aperture. The expression

padding = ( int(ceil((max_extent - shape[0] / 2.))), int(ceil((max_extent - shape[1] / 2.))) )

should be

padding = ( int(ceil((max_extent - (shape[0] - 1) / 2.))), int(ceil((max_extent - (shape[1] - 1) / 2.))) )

@mperrin pointed this error out when I submitted this code originally, and I thought I'd fixed this everywhere, but I apparently overlooked this one.

Opening a PR with the fix!

[kvangorkom]

Fixed in #241.