Add comments to postZephir.pm

[moseshll]

Would be useful to know what each subroutine does, in particular whether it destructively modifies the record.

[aelkiss]

• Deletes PST, LOC, SBL fields (lines 284-286) (shelving location?) - Some records do still include these in Zephir output. Looks like some kind of shelving/holding information.

• Handling for UC "dollar barcodes" - may no longer be needed?

• Update the 974 r,q,t with rights info: lines 370-374

• Update the 974$y with rights date used (line 334)

• Changes the 974$d (for pdus/gfv reversion for downstream processing - hathifiles, full text indexing)

• Replaces non-ascii characters in control fields (00x) (lines 707-720)

• clean_json_line - impenetrable - removes tabs & new lines, replaces non-breaking space with regular space; ensures the indicators are either numeric or blank (or tries to - line 612 might be a bug!); replaces invalid subfields with 'a'. (Shouldn't Zephir do this kind of cleaning...?). Doesn't log any of this cleanup.

• Removes the 974 entirely if the item is suppressed (supp rights code)
  if there are no remaining 974s in the record, then that record should get deleted from the catalog (lines 439-447)

• Check whether the leader character 5 is set to deleted ('d') & if so change it to 'c' (Does this happen in practice? Should Zephir check this?)

• Validates the 008 check_008 but doesn't remediate them (Should Zephir be responsible for this?)

• Validates that the record has a 245 with an a or k subfield (Should Zephir be responsible for this?)

• outputs some stuff about problems with records that we never look at (Maybe Zephir ought to do this..) -- could see whether some of those error-checking things ever happen in practice by reviewing error logs?

• load_prefix_map appears to be unused