API change: remove the Accumulator.SetInitialSeed() method.

Calling Accumulator.SetInitialSeed(), when no seed file is used, is
now no longer required.  Code which used to call .SetInitialSeed()
should be fixed by simply removing this call.

Similarly, seeding Generator objects is no longer required.  The
initial seed of a Generator object is now chosen based on the current
time, the current user name, the currently installed network
interfaces and randomness from the system random number generator.  No
change to existing users of Generator objects is required.