Change probability for an Open Cluster system

Change the success criteria from a 3d roll of 12 or less to a 3d roll
of 10 or less. This results in a new probability of 50%.
Implement generation of stellar characteristics

Generate the stellar characteristics in each star, according to GURPS
Space pages 102-104 (step 18). The only thing missing is finding the
spectral type from the temperature. This will have to be implemented at
a later time.