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