Fix: Prevent zero-division in Cp calculation for zero free-stream velocity

[ayush4874]

Problem

The Pressure Coefficient (Cp) calculation currently divides by ReferenceDynamicPressure without checking if it is zero.
This causes NaN values or crashes when:

1. The reference velocity is set to 0.0 (e.g., in moving reference frame simulations).
2. The user inadvertently sets zero free-stream values.

Solution

Added a safety check for GetReferenceDynamicPressure() in:

• CFlowCompOutput.cpp (Compressible)
• CFlowIncOutput.cpp (Incompressible)
• CNEMOCompOutput.cpp (NEMO / Hypersonic)

If the dynamic pressure is effectively zero (abs < 1e-10), the Cp output defaults to 0.0 instead of performing the division.

[pcarruscag]

Logic for this should be handled in SetReferenceValues of the flow solvers