Implement log units for FITS and VOTable #16540

What is the problem this feature will solve?

The FITS standard allows a number of logarithmic units, namely "log()" and "ln()" as generic units, and "mag" as dimensionless mag unit. While "mag" is supported, "log()" and "ln()" are not.
These units are also not supported on a general level.
At least "log()" could be easily supported, since it seems to be an alias for the supported "dex" magunit (if I understand dex right). And "log" is the one where I now got a user request.

Describe the desired outcome

IMO there should be

• an u.log class which behaves as u.dex (alias) in conversions,
• an extension to the parser to accept log() as a function, like dex(),
• FITS and vounit parser that can handle function units,
• FITS and votable formatter that allow function units to be written, and correctly format the argument (i.e. for vounit the argument shall be vounit compliant).

Like:

>>> import astropy.units as u
>>> u.log(u.cm**-2)
Unit("log(1 / cm2)")
>>> u.log(u.cm**-2).to_string("fits")
'log(1 / cm2)'
>>> u.log(u.cm**-2).to_string("vounit")
'log(cm**-2)'
>>> u.Unit("log(cm**-2)", format="fits")
Unit("log(1 / cm2)")

Additional context

I think that was somewhere discussed before, but I couldn't find the relevant bug. Specifically there was a discussion about why FITS (and VOTable) don't support function units yet.
IMO there was also a can of worms waiting on what the difference between u.mag and u.mag() is, which is a bit confusing.