coordinate transform utils have misleading mnemonics

The word points is used for very different ways of representing 8 coordinates for 4 corners:

• points_from_xywh: a list of integer coordinates
• points_from_x0y0x1y1, xywh_from_points and polygon_from_points: a string with space and comma as delimiters

Can we please rename them to be more precise and consistent? I suggest

• points for the string (because it is also the name of PAGE’s CoordType attribute),
• x0y0x1y0x1y1x0y1 for a list of strings (because it is straightforward and analogous to x0y0x1y1),
• coords or bbox for a list of integers.

Or maybe tltrbrbl instead of x0y0x1y0x1y1x0y1 (but also tlbr instead of x0y0x1y1).

[bertsky]

It will become somewhat better, because all possible conversion options for all the different formats can be seen, and the docstrings are even more precise. But I have not touched the docstrings in ocrd_utils yet.

We now have the following mnemonics:

• polygon – numeric polygon xy-pair list,
• bbox – numeric bbox tuple,
• x0y0x1y1 – string bbox tuple (this name is not ideal),
• xywh – numeric bbox dict,
• points – PAGE polygon xy-pair string

Routines can either convert (without loss of information) or construct (with loss of information or loss of detail).

[kba]

common.py has been merged into core, #268 and #254 should fix this.