Compare two PresenceTypes.
0 will be returned if the types are equal, a positive number will be returned if type_a is more available than 
            type_b, and a negative number will be returned if the opposite is true.
| type_a | the first PresenceType to compare | 
| type_b | the second PresenceType to compare | 
| a number representing the similarity of the two types | 
0.1.11