Allow user-specified target scaling

[carolyner]

Revise the targets object to include target_scale

Use target_scale to scale the distance between the target and the model predicted outcome.

The scale should be used for both selection of draws used for centers and for ordering of draws when narrowing bounds.

[carolyner]

Revise calculation of new target acceptance bounds in get_new_bounds.R, to use the maximum scaled distance across grouped targets.