feat: Implement explicit Tau-Leaping Simulator with Mockito unit tests

[dyrpsf]

Overview

This PR introduces the Explicit Tau-Leaping Algorithm to the FERN simulation engine.

While the Exact Gillespie algorithm is highly accurate, it becomes computationally expensive for networks with massive molecule counts. This new TauLeapingSimulator accelerates stochastic simulations by taking larger time steps ($\tau$) and approximating the number of reaction firings using a Poisson distribution.

Changes Made

• Algorithm Implementation: Added TauLeapingSimulator.java to fern.simulation.algorithm.
• Native Math: Implemented a lightweight, native Poisson random number generator (based on Knuth's algorithm) to avoid bloating the SBSCL dependencies with external math libraries.
• Engine Integration: Successfully interfaced with FERN's AmountManager to correctly process forward reaction firings (performReaction(i, 1)).
• Testing Infrastructure: Added the mockito-core dependency (scope: test) to the root pom.xml to allow for clean, isolated testing of the FERN engine.
• Unit Tests: Created TauLeapingSimulatorTest.java utilizing Mockito to successfully mock the Network and verify simulator initialization and abstract method inheritance.

Validation

• Code compiles successfully (mvn clean compile).
• Unit tests pass locally (mvn clean test -Dtest=TauLeapingSimulatorTest).