Simplify types

[schnellerhase]

Contains multiple changes to the type system.

1. Generalizes dolfinx::scalar concept to allow for general std::complex<T> types (before only float and double supported).
2. Renames trait dolfinx::scalar_value_type to dolfinx::scalar_value and its short notation scalar_value_type_t to scalar_value_t (removes duplicate 'type'). Also renames internal trait variable from value_type to type to highlight different implications.
3. Introduces DofMapSpan type in types.h in favor of multiple definitions of mdspan2_t types. (This caused undefined definition errors when including assembler implementation files)
4. Introduce namespace alias md for MDSPAN_IMPL_STANDARD_NAMESPACE - following style chosen in Simplify mdspan namespace FEniCS/basix#902 (should one globally rename MDSPAN_IMPL_STANDARD_NAMESPACE to md with this?)