Bug 18356: Fix date calculations for yearly frequencies in Serials

Bug 18356: Fix date calculations for yearly frequencies in Serials

The problem as described on BZ 18356 is a combination of an error in
GetFictiveIssueNumber and GetNextDate for unit==year.

[1] In GetNextDate the Add_Delta_YM calculation should be applied only to
frequencies based on years per unit.
In the case of multiple units per year we calculate the number of days to
add. And if we have reached the end of a cycle, we correct the
rounding applied in the cycle.
NOTE 1: We obsolete the idea here of rebasing dates on firstacqui. In case
of manual adjustments, we probably do not want it. And otherwise we do not
need it anymore due to the correction at the end of a cycle.
NOTE 2: The calls to Add_Delta_YM are intentionally not corrected for leap
years. Say you start at 2016-02-29. If you use 1/yr or 1/2yr, you will
switch to the Feb 28th in the following years. In 2020 there will be no
switch to Feb 29 again; if someone should need it, please use a manual
adjustment. This is probably highly exceptional.

[2] In GetFictiveIssueNumber the year should be decreased by one if you
have more units per year and you did not yet reach firstacqui day and
month. This affects calculations in GetNextDate with irregularities.
NOTE 1: I added a wrapper around Date::Calc::N_Delta_YMD in order to improve
its results; this will especially be needed when we use it later for
month units.
NOTE 2: In case of manual adjustments this calculation does not really make
sense. Another report should deal with improving irregularities.

Test plan:
[1] Verify that both GetNextDate.t as well as GetFictiveIssueNumber.t pass.
[2] Look at the prediction pattern for a few frequencies.
    For example: 1 iss/y, 1 iss/2y, 5 iss/y.

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>
(cherry picked from commit d71fb0e17c87a39fc5abe7c9b2d866636c244347)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 07672a77c666354578467c2a35c0f2ea7e512c73)
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>