The method Add adds a new discount.
||Associated array whose keys are the discount parameter names, and the
values are the parameter values.
The following keys are possible:
- LID - the ID of the site to which this discount is bound;
- PRICE_FROM - minimum total order price to which this discount is applicable;
- PRICE_TO - maximum total order price to which this discount is applicable;
- CURRENCY - currency in which all money values are specified in this record;
- DISCOUNT_VALUE - discount value;
- DISCOUNT_TYPE - type of the discount value (P - percentage, V - absolute
- ACTIVE - flag indicating the discount is active (Y/N);
- SORT - sort weight. If more than one discount can be applied to an order price,
the discount with the less SORT is taken first.
Returns the ID of the new discount or false otherwise.