Some changes:

*) Allow to set empty quantity.Discrete objects with certain size
*) added the isConstant property for the quantity.Discrete
*) Implemented a cast to the quantity.Operator object
*) Implemented a cast of empty quantity.Discretes to quantity.Symbolics
*) Fixed bug in "on()" for constant quantities
*) Fixed some functions to work with empty quantities
*) Added a settings object to use global settings
*) plot function returns now an array of the figure handles
*) Re-implementation of the quantity.Operator object