test: Adding more test cases and new lint rules

[albertodev01]

Why?

There are some areas that can be tested more and new lint rules to be applied

What?

• I have upgraded the dart_code_metrics package to 4.6.0 to enable new rules
• Added more unit tests
• Improved the Dart and Flutter examples

Types of Changes

• General improvements (quality updates to improve the stability of the project)

Notes

Bump to a new version (3.1.1)

Checklist

• I have provided a description of the proposed changes.
• I added unit tests for all relevant code.
• I added golden tests for all UI changes.
• I added documentation for all relevant code.

[codecov-commenter]

Codecov Report

Merging #28 (956db28) into master (c33292e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #28   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           66        66           
  Lines         2975      2973    -2     
=========================================
- Hits          2975      2973    -2     

Impacted Files	Coverage Δ
lib/src/algebraic/algebraic.dart	100.00% <100.00%> (ø)
lib/src/algebraic/types/constant.dart	100.00% <100.00%> (ø)
lib/src/algebraic/types/cubic.dart	100.00% <100.00%> (ø)
lib/src/algebraic/types/durand_kerner.dart	100.00% <100.00%> (ø)
lib/src/algebraic/types/linear.dart	100.00% <100.00%> (ø)
lib/src/algebraic/types/quadratic.dart	100.00% <100.00%> (ø)
lib/src/algebraic/types/quartic.dart	100.00% <100.00%> (ø)
lib/src/algebraic/utils/sylvester_matrix.dart	100.00% <100.00%> (ø)
.../src/interpolation/types/linear_interpolation.dart	100.00% <100.00%> (ø)
.../src/interpolation/types/newton_interpolation.dart	100.00% <100.00%> (ø)
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c33292e...956db28. Read the comment docs.