Limits: added new option for a centred cube

[jameswurster]

Currently, to set the limits to plot a centred square or cubic regions, we must manually change each individual limit. This can be tedious during analysis when trying to determine the optimal size, or when we are constantly switching between selected sizes.

This commit adds a new option (numplot+1) where the user enters xyz_max, and the limits are automatically set such that x_max = xyz_max y_max = xyz_max z_max = xyz_max x_min = -xyz_max y_min = -xyz_max z_min = -xyz_max

[danieljprice]

thanks for the contribution!