[FEATURE] add eps-first and eps-decreasing strategy

[thetawom]

Is your feature request related to a problem? Please describe.
It would be great if the library could support other semi-uniform bandit strategies in addition to epsilon-greedy, such as epsilon-first and epsilon-decreasing.

Describe the solution you'd like
Because semi-uniform bandits largely share the same strategy (main difference is how to determine the effective value of epsilon each step), a SemiUniformBandit abstract class implementing the bulk of this strategy can be created. Then, each variant (epsilon-greedy, epsilon-first, epsilon-decreasing) can sub-class SemiUniformBandit.