Bug 29809: Rename item relation accessor from itemnumber

Bug 29809: Rename item relation accessor from itemnumber

This patch renames the item relation accessor method in
StockRotationItem to 'item' from 'itemnumber' to more clearly reflect
that the method returns a Koha::Item object and not an itemnumber.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit a33afd47c11b96e9f13b6d83ca641883c9c70adc)

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>