ode: handling compatiblity of ind var type and scalar types of operators

[fnrizzi]

we currently require:

    && std::is_convertible<
      typename T::independent_variable_type,
      typename ::pressio::Traits<typename T::state_type>::scalar_type
      >::value

and inside we work with scalar_type but i think we should use std::common_t if possible and overall assess how to handle this better. The main issue comes if one wants to use say a custom time type that has, e.g., a builtin unit.