Add tests for central differences for a 2nd order operator with non-zero Dirichlet boundaries

[jlperla]

This expands on #46 to add in the cases where things are non-zero. Not worth doing until a redesign of the interface for boundary values occurs:

• Simple $d X_t = d W_t$ discretization with central differences and with an absorbing boundary at $\underline{x}$ and a reflecting boundary at $\bar{x}$. For the absorbing barrier, make sure to try $\underline{v}\neq 0$
• Simple $d X_t = d W_t$ discretization with central differences and with a reflecting boundary at $\underline{x}$ and an absorbing boundary at $\bar{x}$. For the absorbing barrier, make sure to try $\bar{v}\neq 0$
• Simple $d X_t = \sigma(X_t) d W_t$ discretization with central differences and with an absorbing boundary at $\underline{x}$ and a reflecting boundary at $\bar{x}$. For the absorbing barrier, make sure to try $\underline{v}\neq 0$ and make sure there is variation in the $\sigma(\cdot)$ function.

[jlperla]

We want to do this after #51 is complete.