[mini] Fix stretch factor in Gaussian initialisation w/ spatial and a…

…ngular chirps (#5494)

It seems to me that PR #2173
changed the actual terms of laser initialization in the presence of
chirps. This PR proposes to revert back to the previous behavior, which
I think is more correct. In this way, I think the advantage of
#2173 regarding single-precision
are preserved.

Source/Laser/LaserProfilesImpl/LaserProfileGaussian.cpp

@@ -117,8 +117,8 @@ WarpXLaserProfiles::GaussianLaserProfile::fill_amplitude (
     // Time stretching due to STCs and phi2 complex envelope
     // (1 if zeta=0, beta=0, phi2=0)
     const Complex stretch_factor = 1._rt + 4._rt *
-        (m_params.zeta+m_params.beta*m_params.focal_distance*inv_tau2)
-        * (m_params.zeta+m_params.beta*m_params.focal_distance*inv_complex_waist_2)
+        ((m_params.zeta+m_params.beta*m_params.focal_distance)*inv_tau2)
+        * ((m_params.zeta+m_params.beta*m_params.focal_distance)*inv_complex_waist_2)
         + 2._rt*I*(m_params.phi2-m_params.beta*m_params.beta*k0*m_params.focal_distance)*inv_tau2;
 
     // Amplitude and monochromatic oscillations