Test cases for each purification protocols

[lmpawan10]

Write simulation_tests for each of the purification protocols. SINGLE_SELECTION_X_PURIFICATION is already done at simulation_tests/test_purification_single_x.py. Need similar tests comparing the actual and expected fidelity upon the application of the corresponding purification protocol. Write tests for SINGLE_SELECTION_Z_PURIFICATION, SINGLE_SELECTION_Y_PURIFICATION, SINGLE_SELECTION_XZ_PURIFICATION, SINGLE_SELECTION_ZX_PURIFICATION, DOUBLE_SELECTION_X_PURIFICATION, DOUBLE_SELECTION_Z_PURIFICATION, DOUBLE_SELECTION_XZ_PURIFICATION, DOUBLE_SELECTION_ZX_PURIFICATION, DOUBLE_SELECTION_X_PURIFICATION_SINGLE_SELECTION_Z_PURIFICATION , DOUBLE_SELECTION_Z_PURIFICATION_SINGLE_SELECTION_X_PURIFICATION