Add assert_is_solved_and_feasible

[odow]

Closes #3923

Bikeshedding: the alternative is is_solved_and_feasible(model; assert = true)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 99.52%. Comparing base (7c45f40) to head (d3671ae).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/optimizer_interface.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3925      +/-   ##
==========================================
- Coverage   99.58%   99.52%   -0.07%     
==========================================
  Files          43       43              
  Lines        6059     6063       +4     
==========================================
  Hits         6034     6034              
- Misses         25       29       +4     

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[mlubin]

I like it.