This commit includes changes made to both the Article.php and ArticleList.php classes in the
QUI\ERP\Accounting directory. The setUser methods now accept a null type for the 'User' parameter,
giving more flexibility when calculating user details.

Type hinting was also improved in the ArticleList class. The variable type indicators were changed
from 'int|float' to 'int | float', and from 'Factor|QUI\\ERP\\Products\\Utils\\PriceFactor' to
'Factor | QUI\\ERP\\Products\\Utils\\PriceFactor', improving readability and clarity of code.

Lastly, the 'template' and 'articleTemplate' parameters in the 'toHTMLWithCSS' method now support
both 'bool' and 'string' types, adding flexibility to template usage.

Related: #103

(cherry picked from commit 21a31f8f)

Co-authored-by: Patrick Müller <p.mueller@pcsg.de>

Related: pcsg/buero#344
Related: pcsg/buero#436

feat: An ERP article can now contain custom field data; so it is possible to react to invoice events and further improve certain properties.