fg21sim: v0.6.0

This is a major release with many significant changes since last
release (v0.4.2), so just skip v0.5.x ...

Major changes:

* Rewrite the "extragalactic/clusters" foreground component, with giant
  radio halos considered at the moment.  A physically motivated model is
  adopted to simulate the giant radio halos: cluster mergers generate
  turbulence inside the ICM, which re-accelerate the electrons to make
  them able to radiate synchrotron emissions.

* Rewrite the "sky" module to support the high-resolution simulation
  within sky patches.

* Add "uvsim" sub-module to generate the SKA1-Low layout configuration
  based on the latest specification (v2016-05-31).

* Significantly update the configs/utils modules, as well as their
  users to make them work better.

What works:

* Patch simulation of "galactic/synchrotron", "galactic/freefree".

* Patch simulation of "extragalactic/clusters"

TODO/problems:

* Update "extragalactic/clusters" and tune its parameters to be
  consistent with the observations and existing works.

* Update "galactic/snr" to support patch simulation.

* Update "extragalactic/pointsources" ...

* Update the Web UI against the recent big changes ...

* Update/add the documentations.

* Support radio relics for the "extragalactic/clusters" besides the
  giant radio halos.
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