Prevent multiple additions of the same norm instantiation

[baumfalk]

Currently, an agent can get the same norm instantiation multiple times. I'd say an agent should have a norm instantiation only once. A norm instantiation A is identical to B if

• it has the same norm scheme as B; and
• it has the same agent id as B; and
• it is issued by the same organization as B

An easy solution could be to implement the current norms instantiations for agents as a set and implement the compare function.

[baumfalk]

I don't mean the compare function, but the equals and the hashcode function. See http://stackoverflow.com/questions/27581/what-issues-should-be-considered-when-overriding-equals-and-hashcode-in-java

[baumfalk]

Done!