Allow multiple calls context.initialize()
Allow multiple calls context.initialize()

Subsequent calls to context.initialize() make a new clean
simulation context. The users is warned when they try
to change the fixed execution parameters. MPI, CUDA, etc..
settings are fixed by the first call to context.initialize()

refs #113

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue