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

• Appropriate tests were added
• 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.
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API