OcrdMets.find_files: improve search for pageId

[kba]

Adressing #921:

• regular expression, range and literal values can be combined now, e.g. ocrd workspace find --page-id "PHYS_0001..PHYS_0010,//PHYS_.*[02468],PHYS_9999" will search for
  • PHYS_0001, PHYS_0002 ... PHYS_0010
  • every pageId that starts with PHYS_ and ends with an even number
  • PHYS_9999
• if generating a range:
  • use the last not the first number for the range, because pageIds can have multiple sets of numbers in them. E.g. PHYS_1234_0001..PHYS_1234_0010 did produce a single match PHYS_1234_0001, now it will iterate over the last number part.
  • If the latter case happens, i.e. start equals end number, this will raise a ValueError now.

[bertsky]

Excellent, thanks!