W2Ito1 scheme

[frankschae]

This PR implements the W2Ito1 scheme from Tang, X., & Xiao, A. (2017). Efficient weak second-order stochastic Runge–Kutta methods for Itô stochastic differential equations. BIT Numerical Mathematics, 57, 241-260.

https://link.springer.com/article/10.1007/s10543-016-0618-9

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated -- need to update SDE solver page once it's merged.
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API