`const` version of `data` method is missing in `Array2View`

[thelfer]

Calling data() on an immutable Array2View yields:

error: passing ‘const DataType’ {aka ‘const Arcane::Array2View<double>’} as ‘this’ argument discards qualifiers [-fpermissive]
   61 |     return this->values.data();

• std::span::data is a declared const and returns a pointer to mutable values (which is probably not the semantic adapted for Array2View
• std::vector provides two overloads of the data methods