Schema: Improve evaluation_time property

[kvdblom]

Currently evaluation_time is only defined for Implementations. Since it can vary wildly per problem in an implementation, but also between implementations, it would be nice if:

• evaluation_time became a list for implementations, such that multiple can be given.
• evaluation_time was also added to problems, to have a rough estimate of their cost
• If no evaluation_time is given for a problem, but it has implementation(s), the evaluation_time can be taken from the implementation(s)