Gets the constant change dialog available from the Simulation Control dialog. If varname is missing or empty the list of all constants in the model will be presented. If varname is a valid constant name the constant changes dialog will be brought up for all elements of that constant (if the constant is not subscripted there is only 1 element). This allows the user to change any of the Constants in the model.
Note that if you use GETCNSTCHG on a screen with other value controls (e.g. MODVAR) the values you should follow the GETCNSTCHG command with a SPECIAL>RESETINPUT command to update the screen values.
