ops: revise `fill`

[fnrizzi]

this is just for fill, we need to do these things;

• improve the impl to use PRESSIO_ENABLE_CXX20
• check how setConstat works
• convert explicitly before using
• raise any possible issues about things
• we need to test for when value is of this type :pressio::Traits<T>::scalar_type as well as a type that is really convertible to that (so it has a conversion operator)
• one PR for the impl
• one PR for the documentation (https://pressio.github.io/pressio/components/ops/fill.html)

namespace pressio{ namespace ops{

template <typename T, class ScalarType>
#ifdef PRESSIO_ENABLE_CXX20
 requires (::pressio::is_native_container_eigen<T>::value
    || ::pressio::is_expression_acting_on_eigen<T>::value)
        && std::convertible_to<ScalarType, typename ::pressio::Traits<T>::scalar_type>::value
#else
::pressio::mpl::enable_if_t<
    (::pressio::is_native_container_eigen<T>::value
  || ::pressio::is_expression_acting_on_eigen<T>::value)
  && std::is_convertible<ScalarType, typename ::pressio::Traits<T>::scalar_type>::value
>
#endif 
fill(T & o, const ScalarType & value)
{
  const typename ::pressio::Traits<T>::scalar_type newVal{value};
  impl::get_native(o).setConstant(newValue);
}

}}//end namespace pressio::ops