Change of image_center from pixel to data coordinates.

Hopefully this will (at least partially) fix issue #1062. If nothing
else, it will allow the centre of rotation to be specified as outside
the image.
Partial working rotate fixes.
Fixed rotation of image data
Stopped passing of NaNs to scipy's affine_transform
Fixed recenter test
Change the .T from the skimage to the scipy.

This means that the affine_transform routine rotates a C ordered
numpy array in the correct direction. This breaks FITS files.
Fix the submap thing while keeping affine_transform rotating CCW.

This however might break non-map recentre calls to affine_transform.
Flip array_center so that it matches image_center.

For some reason only @drewleonard42 knows the image_center argument to
affine_transform is (x, y) rather than (y, x). This should be fixed at a
later date.
Add SciPy NaN warning and doc.
Fixed unintended modification of input image
change map_centre to array_centre.
Syntatix niceness in tests.
map.rotate doc string improvements.

Use See Also to link affine_transform and make it clear that scipy and
skimage are not the same thing and point out that higher order
interpolation is more accurate but slower.
Further documentation improvements.
Add explanation of defualt order to affine_transform.
Merge pull request #1074 from Cadair/rotate_fix

[Review]: Rotation fixes for non-centered maps, submaps, and skimage/scipy consistency