Re-work how solutions are reported about powermodels.jl

[ccoffrin]

A key feature that is needed is a way to support using multiple variables. For example, when computing voltage magnitude from rectangular coordinates.

[ccoffrin]

See if multiple dispatch and generic algorithms can be used to improve this.

[ccoffrin]

One possibility, a generic solution builder walks var and con and makes a data dict out of the values in there. Formulations can add expressions / functions in var / con to derive different values on solution reporting. By default everything will be reported but formulations can specify specific component/value pairs that are allowed.

[ccoffrin]

A prototype solution has been developed in the sol-rev2 branch. Additional required features include,

• components ids are required in sol_post functions
• sol_post functions should be given as sets or lists, not Dicts as the keys are not relevant
• foundational tools should be abstracted into InfrastructureModels