Add 'order' kwarg for interpolation in profile_line()
Polish coordinate system

It's important to distinguish between pixels and points. A square set
of points belongs to one pixel. When users type `linewidth=3`, they
usually mean a line 3 pixels wide. The distance between the pixel
center points, then, is 2.
Add numerous (14) tests for profile_line()
Remove unnecessary if clause

The `if linewidth <= 1` was necessary because the coordinate systems
(pixel/point) were all muddled up. Now that it has been clarified,
`linewidth=1` works transparently!
Change default interpolation order to 1