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• Sanity checks to make sure results aren't queried with an out-of-date model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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

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

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