Fix data types in boundary handling. Fix deprecation checks.

Fix some data-type issues in boundary handling discovered while patching lbmpy.

• Adapt add_subexpressions_for_field_reads to insert explicit casts when a nondefault data type is required
• Permit modifying the default data type in BoundaryHandling
• Fix target check in handling deprecated vectorization options

src/pystencils/simp/simplifications.py

+from __future__ import annotations
+from typing import TYPE_CHECKING
+
 from itertools import chain
 from typing import Callable, List, Sequence, Union
 from collections import defaultdict
 
 import sympy as sp
 
+from ..types import UserTypeSpec
 from ..assignment import Assignment
-from ..sympyextensions import subs_additive, is_constant, recursive_collect
+from ..sympyextensions import subs_additive, is_constant, recursive_collect, tcast
 from ..sympyextensions.typed_sympy import TypedSymbol
 
+if TYPE_CHECKING:
+    from .assignment_collection import AssignmentCollection
+
 
 # TODO rewrite with SymPy AST
 # def sort_assignments_topologically(assignments: Sequence[Union[Assignment, Node]]) -> List[Union[Assignment, Node]]:
@@ -170,14 +177,19 @@ def add_subexpressions_for_sums(ac):
     return ac.new_with_substitutions(substitutions, True, substitute_on_lhs=False)
 
 
-def add_subexpressions_for_field_reads(ac, subexpressions=True, main_assignments=True, data_type=None):
+def add_subexpressions_for_field_reads(
+    ac: AssignmentCollection,
+    subexpressions=True,
+    main_assignments=True,
+    data_type: UserTypeSpec | None = None
+):
     r"""Substitutes field accesses on rhs of assignments with subexpressions
 
     Can change semantics of the update rule (which is the goal of this transformation)
     This is useful if a field should be update in place - all values are loaded before into subexpression variables,
     then the new values are computed and written to the same field in-place.
     Additionally, if a datatype is given to the function the rhs symbol of the new isolated field read will have
-    this data type. This is useful for mixed precision kernels
+    this data type, and an explicit cast is inserted. This is useful for mixed precision kernels
     """
     field_reads = set()
     to_iterate = []
@@ -201,8 +213,23 @@ def add_subexpressions_for_field_reads(ac, subexpressions=True, main_assignments
             substitutions.update({fa: TypedSymbol(lhs.name, data_type)})
         else:
             substitutions.update({fa: lhs})
-    return ac.new_with_substitutions(substitutions, add_substitutions_as_subexpressions=True,
-                                     substitute_on_lhs=False, sort_topologically=False)
+    
+    ac = ac.new_with_substitutions(
+        substitutions,
+        add_substitutions_as_subexpressions=False,
+        substitute_on_lhs=False,
+        sort_topologically=False
+    )
+
+    loads: list[Assignment] = []
+    for fa in field_reads:
+        rhs = fa if data_type is None else tcast(fa, data_type)
+        loads.append(
+            Assignment(substitutions[fa], rhs)
+        )
+
+    ac.subexpressions = loads + ac.subexpressions
+    return ac
 
 
 def transform_rhs(assignment_list, transformation, *args, **kwargs):