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

tests/generator_scripts/source/JacobiMdspan.py

@@ -22,6 +22,6 @@ with SourceFileGenerator() as sfg:
     sfg.function("jacobi_smooth")(
         sfg.map_field(u_src, mdspan.from_field(u_src, layout_policy="layout_left")),
         sfg.map_field(u_dst, mdspan.from_field(u_dst, layout_policy="layout_left")),
-        sfg.map_field(f, mdspan.from_field(f)),
+        sfg.map_field(f, mdspan.from_field(f, layout_policy="layout_left")),
         sfg.call(poisson_kernel)
     )