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.harness.cpp

@@ -5,21 +5,21 @@
 
 namespace stdex = std::experimental;
 
-using field_t = stdex::mdspan<double, stdex::extents< int64_t, std::dynamic_extent, std::dynamic_extent>>;
-using scalar_field_t = stdex::mdspan<double, stdex::extents< int64_t, std::dynamic_extent, std::dynamic_extent, 1>>;
+using field_t = stdex::mdspan<double, stdex::extents<int64_t, std::dynamic_extent, std::dynamic_extent>, stdex::layout_left>;
+using scalar_field_t = stdex::mdspan<double, stdex::extents<int64_t, std::dynamic_extent, std::dynamic_extent, 1>, stdex::layout_left>;
 
 int main(void)
 {
     auto data_f = std::make_unique<double[]>(64);
-    scalar_field_t f{ data_f.get(), 8, 8 };
+    scalar_field_t f{data_f.get(), 8, 8};
 
     auto data_u = std::make_unique<double[]>(64);
-    field_t u{ data_u.get(), 8, 8 };
+    field_t u{data_u.get(), 8, 8};
 
     auto data_u_tmp = std::make_unique<double[]>(64);
-    field_t u_tmp{ data_u_tmp.get(), 8, 8 };
+    field_t u_tmp{data_u_tmp.get(), 8, 8};
 
-    double h { 1.0 / 7.0 };
+    double h{1.0 / 7.0};
 
     gen::jacobi_smooth(f, h, u_tmp, u);
 }