Add variable is `Solving` state && free sol after adding

[codecov[bot]]

Codecov Report

Merging #100 (45964b4) into development (3659358) will decrease coverage by 2.17%. Report is 7 commits behind head on development. The diff coverage is 94.42%.

:exclamation: Current head 45964b4 differs from pull request most recent head abe42e8. Consider uploading reports for the commit abe42e8 to get more accurate results

@@               Coverage Diff               @@
##           development     #100      +/-   ##
===============================================
- Coverage        91.19%   89.02%   -2.17%     
===============================================
  Files               12       13       +1     
  Lines              795      829      +34     
===============================================
+ Hits               725      738      +13     
- Misses              70       91      +21     

Files Changed	Coverage Δ
src/branchrule.rs	66.67% <ø> (ø)
src/heuristic.rs	100.00% <ø> (ø)
src/lib.rs	50.00% <ø> (+50.00%)	:arrow_up:
src/node.rs	100.00% <ø> (ø)
src/pricer.rs	75.00% <ø> (ø)
src/solution.rs	100.00% <ø> (ø)
src/model.rs	94.76% <90.79%> (-3.30%)	:arrow_down:
src/scip.rs	95.02% <95.02%> (ø)