Bug 10306: Restructure TransformMarcToKoha and update some POD lines

Bug 10306: Restructure TransformMarcToKoha and update some POD lines

This patch makes the following changes:

[1] Added POD for CountItemsIssued, GetBiblioItemData
[2] Moved TransformMarcToKohaOneField closer to TransformMarcToKoha (before
    sub CountItemsIssued)
[3] Restructured TransformMarcToKoha by extracting individual kohafields via
    TransformMarcToKohaOneField. The latter does no longer call the former.
    This small optimization avoids traversing the whole MARC record again
    and again.
[4] Moved adjusting copyrightdate/publicationyear to separate helper routine
    _adjust_pubyear
[5] Removed obsolete sub _get_inverted_marc_field_map.

Test plan:
Run t/db_dependent/Biblio/TransformMarcToKoha.t

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>