don't replace particle with star in stromgren propagation tests

[Note that this MR isn't for the master branch, but the SPHM1RT_injection branch]

This version doesn't replace a gas particle in the stromgren sphere propagation tests, but adds the star somewhere close to the center.

Advantage: particles should have uniform density now.

Disadvantage: injected flux and energy may result anisotropic now:

Feel free to reject this MR if it's not to your liking, TK