Option to allow spherical star and other simplifications

[thjsal]

For testing and code comparison purposes, it can be handy to have the option to calculate the pulses from a spherical neutron star (instead of an oblate).

Here are instructions on how to make the star spherical currently in X-PSI:

Make this line:

xpsi/xpsi/cellmesh/mesh_tools.pyx

Line 20 in 7627389

return 1.0 + epsilon * (-0.788 + 1.030 * zeta) * mu * mu

to return 1.0.

Make this line:

xpsi/xpsi/cellmesh/mesh_tools.pyx

Line 31 in 7627389

return radiusDerivNormed / (radiusNormed * sqrt(1.0 - 2.0 * zeta / radiusNormed))

to return 0.0.

Make this line:

xpsi/xpsi/surface_radiation_field/effective_gravity_universal.pyx

Line 32 in 7627389

return log10(g * g_0) + 2.0

to return log10(g_0) + 2.0.

And then re-install X-PSI.

We can also think if turning time delays off could be a useful option. That can be turned off currently by setting _phase_lag=0.0 in the integrator files found in https://github.com/xpsi-group/xpsi/tree/main/xpsi/cellmesh and then re-installing X-PSI.