Extend mdspan interface and fix mdspan memory layout mapping

This MR fixes the way memory layouts for std::mdspan are modelled and inferred.

• Add the layout_policy template parameter to the StdMdspan class.
• Use layout_strided by default (cannot infer layout from field instance, because of pystencils#117)
• Extend mdspan AugExpr interface
• Extend mdspan documentation
• Add test cases for checking shape and strides of mdspans created from fixed-shape fields
• Add test cases for checking shape and strides of mdspans created from var-shape fields

src/pystencilssfg/lang/cpp/std_mdspan.py

@@ -160,9 +160,8 @@ class StdMdspan(SrcField):
 
         return Extraction()
 
-    @classmethod
+    @staticmethod
     def from_field(
-        cls,
         field: Field,
         extents_type: UserTypeSpec = PsUnsignedIntegerType(64),
         layout_policy: str | None = None,