fg21sim: v0.6.3

* Rewrite turbulent acceleration timescale calculation, which now gives more
  reasonable acceleration efficiency (~100 Myr), while the original
  calculation was a wrong over-simplification to the [cassano2005]'s formula,
  leading to almost no acceleration at all (~10 Gyr).

  A new parameter ``f_acc`` is added to tune the acceleration timescale as
  well.

* A more sophisticated method is implemented to derive the initial electron
  spectrum for halos, which is derived from the accumulated injected electron
  spectrum by considering a period of evolution (``time_init``) with only
  energy losses and constant injection (which is minor).

  This improvement gives more reasonable halo spectral index and acceleration
  results.

* Rewrite the ICM temperature calculation method, replacing the original M-T
  scaling relation with the virial temperature.  The new parameter ``kT_out``
  is added to account the additional shock heating.

* Replace the magnetic field strength calculation by assuming its energy
  density is a fraction (``eta_b``) of the thermal energy density.

* Support use the maximum merger events instead of the major merger events,
  and the parameter ``use_max_merger`` is added to use this.

* Fix the halo profile calculation, which original wrongly use the halo radius
  as the e-folding radius, causing a 2 times larger halos in the simulated
  images.

* Fix the passing of ``configs`` object to the helper functions.

* Various tests carried out to tune the parameters, and the default parameters
  values are updated.

* Many small tweaks and improvements.
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