* Update "output()" method to return the output file path
* Update "simulate()" to also return the list of output file paths

Since some modules (e.g., healpix.py) under "utils" involves Numba JIT
optimization, which significantly slows the start-up speed, even some
consumers (e.g., bin/fg21sim-webui) do not need them at all.

Fix various import statements of other modules.

NOTE: not test at the moment!

Also add a warning log message for "galactic/snr.py" and
"extragalactic/clusters.py".

When two quantities of same unit class divides, to get the correct
numerical value, whether use `.decompose().value`, or just convert to
plain Python object, e.g., `float(...)`, which will fail if the input
quantity is not dimensionless.

* freefree.py: Add the missing "/" to the config "halphamap"
* synchrotron.py: Fix "comp" to have the correct value

* Add the "preprocess()" and "postprocess()" for a better interface;
* Add "_make_filepath()" to compose the output filename;
* Reorder some methods for consistency.

* Add the "preprocess()" and "postprocess()" for a better interface;
* Add "_make_filepath()" to compose the output filename;
* Reorder some methods for consistency;
* Some refactorings and cleanups.

* Add "preprocess()" and "postprocess()" methods
* Add the "name" property
* Tweak the places of preparation procedures been invoked
* Reorder some methods