FG21sim: v0.6.4

* Estimate the radio halo radius to be the turbulence injection scale:
  ``f_lturb*R_vir`` (``f_lturb`` default to be 0.33).

* Calculate the turbulence Mach number (i.e., velocity dispersion) based on
  the merger energy, thus more severe merger (i.e., smaller M_main/M_sub
  ratios) resulting in higher turbulence Mach number (~0.2-0.5) and thus more
  powerful/brighter radio halos.

* Rework the turbulence acceleration timescale calculation, which is simpler
  and more intuitive.

* Tune the maximum turbulence acceleration timescale to 10 Gyr for a more
  consistent simulation results at higher redshifts.

* Various small updates/tweaks and cleanups.

Happy New Year!
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