Boxed plugins & Model Clone

[mmghannam]

Changes:

• Use Box<dyn PLUGIN> instead of &mut PLUGIN. This fixes the issue that the plugin's trait object data was lost when transferring to and from SCIP's call back.
• Add clone_for_plugins method for Model to allow calling SCIP functions within plugins. This is to replace the ModelRef struct that was causing undefined behavior probably because of the raw pointer dereferencing.

[codecov[bot]]

Codecov Report

Merging #71 (96438c2) into development (1b3875b) will increase coverage by 0.70%. The diff coverage is 87.80%.

@@               Coverage Diff               @@
##           development      #71      +/-   ##
===============================================
+ Coverage        72.83%   73.53%   +0.70%     
===============================================
  Files                9        9              
  Lines              552      578      +26     
===============================================
+ Hits               402      425      +23     
- Misses             150      153       +3     

Impacted Files	Coverage Δ
src/branchrule.rs	55.56% <ø> (ø)
src/node.rs	100.00% <ø> (ø)
src/pricer.rs	75.00% <ø> (ø)
src/model.rs	83.83% <87.80%> (+0.30%)	:arrow_up: