Write points fix

[will-moore]

This unifies write_table_regions() and write_table_points() etc to all use the fixes that I'd previously only applied to write_table_regions().

I noticed that write_table_circles() was using @type:

table_group.attrs["@type"]

but everywhere else this was simply table_group.attrs["type"]. This will now use type everywhere.

[will-moore]

I realise this is used by openssbd/bdz#3 and contains useful fixes, so re-opening...
(although I understand that the NGFF tables spec is still undecided and all may change once that is finalised)