Add option callback

[keurfonluu]

• Fixed: zero division error in NA when upper and lower bounds are equal,
• Added: option callback executed after every iteration.

Reminders:

• [x] Run invoke format to make sure the code follows the style guide,
• [x] Add tests for new features or tests that would have caught the bug that you're fixing,
• [x] Write detailed docstrings for all functions, classes and/or methods,
• [x] If adding new functionality, unit test it and add it to the documentation.

[codecov-commenter]

Codecov Report

Merging #12 (96c8d40) into master (682a601) will decrease coverage by 0.99%. The diff coverage is 90.85%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage   90.06%   89.06%   -1.00%     
==========================================
  Files          31       31              
  Lines        1037     1116      +79     
==========================================
+ Hits          934      994      +60     
- Misses        103      122      +19     

Impacted Files	Coverage Δ
stochopy/optimize/pso/_pso.py	100.00% <ø> (ø)
stochopy/sample/_common.py	80.00% <50.00%> (ø)
stochopy/optimize/na/_na.py	88.11% <83.78%> (-3.55%)	:arrow_down:
stochopy/optimize/vdcma/_vdcma.py	86.90% <84.61%> (-0.75%)	:arrow_down:
stochopy/optimize/cpso/_cpso.py	88.33% <87.09%> (-1.58%)	:arrow_down:
stochopy/optimize/de/_de.py	88.67% <87.09%> (-1.85%)	:arrow_down:
stochopy/sample/hmc/_hmc.py	86.73% <90.00%> (-1.22%)	:arrow_down:
stochopy/optimize/cmaes/_cmaes.py	89.20% <96.00%> (-1.02%)	:arrow_down:
stochopy/sample/mcmc/_mcmc.py	91.04% <96.42%> (+0.30%)	:arrow_up:
stochopy/factory/benchmark.py	100.00% <100.00%> (ø)
... and 6 more

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