Allow to round taxes on order-level (#5019)

* Allow to round taxes on order-level

* Rename get_cart_total

* Persist rounding mode with order

* Add general docs

* Order creation API

* Update fee algorithm

* Rounding on payment method change

* Round when splitting order

* Fix failing tests

* Add settings page

* Add tests

* Replace algorithm

* Add test case for currency rounding

* Improve order change

* Update flowchart

* Update discount logic (more hypothetical, we don't store rounding on cart positions atm)

* Rename internal method

* Fix typo

* Update help text

* Apply suggestions from code review

Co-authored-by: luelista <weller@rami.io>

* Order rounding refactor (#5571)

* Add RoundingCorrectionMixin providing before-rounding-values as properties

* Use gross_price_before_rounding in more places

* Update doc/development/algorithms/pricing.rst

Co-authored-by: Martin Gross <gross@rami.io>

* Allow to override on perform_order

* Rebase migration

* Fix event cancellation

---------

Co-authored-by: luelista <weller@rami.io>
Co-authored-by: Martin Gross <gross@rami.io>

src/pretix/base/settings.py

@@ -77,6 +77,13 @@ from pretix.control.forms import (
 )
 from pretix.helpers.countries import CachedCountries
 
+ROUNDING_MODES = (
+    ('line', _('Compute taxes for every line individually')),
+    ('sum_by_net', _('Compute taxes based on net total')),
+    ('sum_by_net_keep_gross', _('Compute taxes based on net total with stable gross prices')),
+    # We could also have sum_by_gross, but we're not aware of any use-cases for it
+)
+
 
 def country_choice_kwargs():
     allcountries = list(CachedCountries())
@@ -324,7 +331,7 @@ DEFAULTS = {
         'form_class': forms.BooleanField,
         'serializer_class': serializers.BooleanField,
         'form_kwargs': dict(
-            label=_("Show net prices instead of gross prices in the product list (not recommended!)"),
+            label=_("Show net prices instead of gross prices in the product list"),
             help_text=_("Independent of your choice, the cart will show gross prices as this is the price that needs to be "
                         "paid."),
 
@@ -465,6 +472,25 @@ DEFAULTS = {
             widget=forms.CheckboxInput(attrs={'data-checkbox-dependency': '#id_settings-order_phone_asked'}),
         )
     },
+    'tax_rounding': {
+        'default': 'line',
+        'type': str,
+        'form_class': forms.ChoiceField,
+        'serializer_class': serializers.ChoiceField,
+        'form_kwargs': dict(
+            label=_("Rounding of taxes"),
+            widget=forms.RadioSelect,
+            choices=ROUNDING_MODES,
+            help_text=_(
+                "Note that if you transfer your sales data from pretix to an external system for tax reporting, you "
+                "need to make sure to account for possible rounding differences if your external system rounds "
+                "differently than pretix."
+            )
+        ),
+        'serializer_kwargs': dict(
+            choices=ROUNDING_MODES,
+        ),
+    },
     'invoice_address_asked': {
         'default': 'True',
         'type': bool,