Adding function checkmodel.

[bbejanov]

Model checks done in @initialize are @reinitialize are now done in a new function checkmodel. This reduces code duplication.

Also, adding a new check that model has the saem number of equations and (non shock, non exog) variables.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.10%. Comparing base (344efd3) to head (e5e34bb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   95.64%   95.10%   -0.54%     
==========================================
  Files          14       14              
  Lines        2203     2206       +3     
==========================================
- Hits         2107     2098       -9     
- Misses         96      108      +12     

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