Normal shock mode : better error handling required when illegal values are entered

[ashwinvis]

Should not get core dumped. Will have to be dealt in scikit-aero/skaero/gasdynamics/isentropic.py or in caeroc.

DEBUG:caeroc-app:MODE: Normal shock relations
INFO:caeroc-app:{'M_1': 0.9}
CRITICAL:caeroc-app:Mach number must be supersonic
Traceback (most recent call last):
  File "/scratch/avmo/opt/caeroc-r8bspf19/src/scikit-aero/skaero/gasdynamics/isentropic.py", line 54, in mach_angle
    mu = np.arcsin(1 / M)
FloatingPointError: invalid value encountered in arcsin

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/avmo/src/caeroc/caeroc/gui/runtime.py", line 214, in on_qpb_calculate_released
    self.mode.calculate(**kwargs)
  File "/home/avmo/src/caeroc/caeroc/formulae/shock.py", line 89, in calculate
    self.store(key, getattr(self, key))
  File "/scratch/avmo/opt/caeroc-r8bspf19/src/scikit-aero/skaero/gasdynamics/shocks.py", line 219, in M_2
    M_2 = self.M_2n / np.sin(self.beta - self.theta)
  File "/scratch/avmo/opt/caeroc-r8bspf19/src/scikit-aero/skaero/gasdynamics/shocks.py", line 194, in theta
    if self.beta == mach_angle(self.M_1) or self.beta == np.pi / 2:
  File "/scratch/avmo/opt/caeroc-r8bspf19/src/scikit-aero/skaero/gasdynamics/isentropic.py", line 56, in mach_angle
    raise ValueError("Mach number must be supersonic")
ValueError: Mach number must be supersonic
[1]    11343 abort (core dumped)  caeroc-app