Improving print to use scientific formatting

[mariohsouto]

This PR makes two changes to the code:

1. Improve print by using scientific convention with two decimals,
2. Fix Model(with_optimizer issue.

[codecov]

Codecov Report

Base: 82.39% // Head: 82.39% // No change to project coverage 👍

Coverage data is based on head (458130a) compared to base (a60f3da).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #93   +/-   ##
=======================================
  Coverage   82.39%   82.39%           
=======================================
  Files          11       11           
  Lines        1267     1267           
=======================================
  Hits         1044     1044           
  Misses        223      223           

Impacted Files	Coverage Δ
src/printing.jl	91.53% <100.00%> (ø)

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