Add covariance_matrix output to InjectorSurrogate

[bhardwaj-gopika]

Derive a diagonal 6x6 covariance matrix from the scalar beam parameters (XRMS, YRMS, sigma_z, norm_emit_x, norm_emit_y) with no off-diagonal terms. Energy is a constructor parameter for normalized-to-geometric emittance conversion.

this can be used like so

model = InjectorSurrogate(energy=135e6)  
model.set({"QUAD:IN20:525:BCTRL": -5.0})
cov = model.get(["covariance_matrix"])["covariance_matrix"]  # 6x6 np.ndarray