Add fixtures for setting default values to structs.
Add fixtures for setting default values to structs.

- Three fixtures, each for rpacket, storage model
  and mt_state were added in test_cmontecarlo.py

- These fixtures have the same declaration as it
  was in init methods of corresponding C tests.

Tobychev/tardis,kaushik94/tardis,Tobychev/tardis,orbitfold/tardis,kaushik94/tardis,kaushik94/tardis,Tobychev/tardis,orbitfold/tardis,kaushik94/tardis,orbitfold/tardis,orbitfold/tardis