get_uint64
Description:
Gets the value that is stored at key in this.
A convenience variant of @get for 64-bit unsigned integers.
It is a programmer error to give a key that isn't specified as having a uint64 type in the schema for 
        this.
Parameters:
| this | a Settings object | 
| key | the key to get the value for | 
Returns:
| a 64-bit unsigned integer |