Backend API Refinements

[pulsipher]

This is a continuation of #349 . This comprises a number of refinements to the transformation API as motivated by InfiniteExaModels. Improvements include:

• A reduction in the methods that need to be extended for queries
• The addition of JuMP.is_solved_and_feasible
• Sanity checks to make sure results aren't queried with an out-of-date model
• Added public API functions that are helpful for extensions (i.e., parameter_group_indices, parameter_refs(::InfiniteModel), parameter_group_int_index, all_expression_variables, core_object)
• Change of nomenclature from "object numbers" to "infinite parameter group integer indices".
• Code base simplifications

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.77%. Comparing base (db449cc) to head (179a077).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #351 +/- ## ======================================= Coverage 99.77% 99.77% ======================================= Files 35 35 Lines 6712 6758 +46 ======================================= + Hits 6697 6743 +46 Misses 15 15 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.