support multiple POINTS conventions

[bertsky]

https://github.com/kba/page-to-alto/blob/081dbb4c612214ce321b166f7661c5bbd563d8df/ocrd_page_to_alto/utils.py#L20

This implements the PAGE syntax for @points directly as ALTO @POINTS. But IIUC a more common convention uses spaces instead of commas. (See discussion here.)

We should add another CLI parameter for the actual conventions used in the wild.