Bug 18697: Final polishing

Bug 18697: Final polishing

GetFictiveIssueNumber:
Returns undef instead of 0 for irregular frequencies. Also added to POD.
Removed unused variable $wkno.
Adding a return makes the if(unit) unneeded.
Replaced (a+b)/b by 1+a/b.

_delta_units:
Added a comment about its parameters.

GetFictiveIssueNumber.t:
Adjusted the tests for irregular frequencies accordingly.

Test plan:
[1] Run t/db_dependent/Serials/GetFictiveIssueNumber.t
[2] Run t/db_dependent/Serials/GetNextDate.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
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>