FG21sim: v0.9.0

* Significantly improve the modeling of radio halos.

  - More reasonable estimation of the turbulence region size (relating
    with the stripping radius of the in-falling sub-cluster).
  - Improved calculation of merger-induced turbulence velocity
    dispersion (i.e., Mach number) and acceleration velocity.
  - More configuration parameters to fine-tune the correlation
    assumptions in the model.
  - Fine-tuned parameters against observations and previous works
    (observational correlations, simulation/predictions).

* Better implemention of the halo modeling, including many major updates
  to the code.

* Many cleanups and minor improvements.

* Update the documentations.

* Remove the obsolete code for point sources.

This is a very significant update over the last v0.7.2, so skip 0.8.x.
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