Add support for premium overrides

You can change how PolicyCenter handles premiums for a policy line to support overriding the premium that the rating engine calculates.

Procedure

1. Check if there is an existing cost.
2. If so, check if there are non-null values for any override properties.
3. If there are overrides, copy the appropriate properties from the existing cost.
4. Find and use override properties in your rate table lookups and algorithms.
5. Optionally, disallow user overrides for a cost by setting the Overridable property on a new cost data object to false. The default value of Overridable is true.
6. Decide how you want to handle zero-cost costs. A real-world rating engine calculates that there is zero cost ($0) for a particular cost in some cases. Typically, the rating engine does not generate a cost row for that zero cost. Because the premium overrides user interface allows you to override existing rows but not add new Cost rows, by default, you cannot override a zero-cost cost. To allow users to edit (override) this cost, you can choose to create a zero-cost Cost entity instance in these cases. If you want to do this, you must modify the quote screen to filter out all the zero-cost rows. To edit those rows, you must show these zero-cost costs in the premium override screen.