CarpetX: Should boundary parameters be renamed to boundary_lower?

[SamuelCupp]

As discussed in today's call, the boundary_x, boundary_y, and boundary_z parameters only set the lower boundaries. This is due to historical reasons relating to reflection boundaries. However, it might make more sense to have boundary_lower.

This naming convention is present in all the boundary parameter names (reflection_x and reflection_upper_x, linear_extrapolation_x_vars and linear_extrapolation_upper_x_vars, etc).