Link caeroc.formulae.shock to caeroc.gui.runtime

[ashwinvis]

Allow user to use Normal Shock and Oblique Shock modes.

[ashwinvis]

Bug: cannot use p2_p1 with normal shock mode.

ERROR:caeroc-app:Insufficient data to calculate Mach number
Traceback (most recent call last):
  File "/home/avmo/src/caeroc/caeroc/gui/runtime.py", line 218, in on_qpb_calculate_released
    self.mode.calculate(**kwargs)
  File "/home/avmo/src/caeroc/caeroc/formulae/shock.py", line 81, in calculate
    self.M_1 = self.get_M_1(p2_p1, rho2_rho1, T2_T1)
  File "/home/avmo/src/caeroc/caeroc/formulae/shock.py", line 59, in get_M_1
    return M_1
UnboundLocalError: local variable 'M_1' referenced before assignment