Add reference energy to problem instance schema

[jtcantin]

Current problem instance schema does not include reference energies explicitly.
After 10-Dec, it would be good to add this to the schema, with the following features:

• Have a benchmark_instance flag
• If the benchmark_instance flag is true, then a reference energy is required. If not, such as for guidestar instances, the reference energy is not required. We can use the same approach as used for solution files and quantum resource estimates, in relation to a calculated energy.
• Have a required (?) source string or object with information on the reference energies.

[jp7745]

folding this issue in with #54 .