25 weightset

[ikfir]

Close #25

[liorsl]

2 things that I fixed:

• there was no documentation on the class
• better to use Assertions class rather than assert for unit tests

Missing unit tests for the following methods:

• [x] WeightSet::new(Map)
• [x] WeightSet#getWeightOrDefault
• [x] WeightSet#getPercentage