Draft: Added Smoothed RT option to SPHM1RT

Added a new injection scheme, namely smoothedRT:

If smoothedRT is set to 1, we use the Smoothed RT scheme (see Section 4.4 in doi:10.1093/mnras/stt1722). Instead of adding radiation energy and then decaying through thermo-chemistry, we add source terms for radiation in the thermo-chemistry equations. Smoothed RT should be faster because radiation is not artifically boosted and decayed within the time-step. Smoothed RT should also be more accurate since it models the physical process more accurately -- radiation is continously injected and interacts with matter.

Test with Stromgren_3D singlebin and the result is satisfactory: