Small adjustments to GEAR modules

This MR implements a few things:

• I added y name to the GEAR/sink files to show that these files changed this year compared to Loic's 2021 sink version
• I updated my email address from epfl to alumni.epfl. The latter is a lifetime mail address that redirects where I want it. Hence, updating my current UNIGE account in four years is unnecessary.
• In the GEAR sink, I updated the density threshold units from g/cm3 to H/cm3, which are more convenient. (as we discussed a few weeks ago)
• In GEAR SF, I added units to the names of the temperature and density thresholds and changed the units of the density threshold from g/cm3 to H/cm3 for the same reason. This is now consistent with the sink module.
• The last two changes lead me to update all examples with GEAR SF and sinks I made the IsolatedGalaxy_sink example work with GEAR sinks, added a README, and added a symlink from the sink examples.
• Since this example has stars in the ICs, and because many ICs can be downloaded to play with, I tried implementing #900 (closed) (I'm not sure I like this implementation).

There may be too many changes at once. The most critical is in src/io_properties.h. Let me know if you prefer to split this into another MR.

TODO:

• Have I updated the doc?
• Check all X_io.h files to verify we pass addressable variables