Update Mengaldo BCs

[tulioricci]

Add no-shear stress condition for Symmetry/slip wall
Add RiemannInflow,
Add linearized Euler Eqs. for outflow
Rename OutflowBoundary to PressureOutflowBoundary

Questions for the review:

• Is the scope and purpose of the PR clear?
  • The PR should have a description.
  • The PR should have a guide if needed (e.g., an ordering).
• Is every top-level method and class documented? Are things that should be documented actually so?
• Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• Does the implementation do what the docstring claims?
• Is everything that is implemented covered by tests?
• Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?

[MTCam]

How's come this? Why not s'th like:

Suggested change

	self._ref_state = free_stream_state
	self._mass = free_stream_density
	self._velocity = free_stream_velocity
	self._pressure = free_stream_pressure
	self._temperature = free_stream_temperature
	self._Y = free_stream_species_mass_fractions
	if free_stream_state is not None:
	self._mass = free_stream_state.mass
	(...) etc

... and then eliminate the if/else blocks that check for existence of self._ref_state?