get_uint64
Description:
Sets the uint64 pointed to by value corresponding to the value of the given 
      field.
Caller is responsible for making sure the field exists and has the correct type.
Parameters:
| this | |
| fieldname | the name of a field | 
| value | a pointer to a uint64 to set | 
Returns:
| true if the value could be set correctly. If there was no field with  |