Copyright years: updated to 2023, accompanied by some minor fixes

SUMMARY: just house-cleaning, no need to do anything major in your
branch. Just update the copyright years in files that you have added.

Until now, the latest copyright years of the whole Maneage source code was
2022! As of this commit, we have already moved to 2023 for 5 months!
Furthermore, there were a few other minor issues that needed correction:

 - The URL to download input datasets wasn't quoted in 'initialize.mk' or
   the download script! As a result, when the input URL had characters that
   are meaningful to the shell (like '&'), the download command would not
   work.

 - The only program that had 'make check' in the 'basic.mk' programs was
   MPFR. At that stage, we still haven't built our own compiler at this
   stage, this is not accurate.

 - The 'pyerfa' and 'extension-helpers' packages in Python need
   'setuptools_scm' on some systems. But until now, it was not in the list
   of their prerequisites.

With this commit, all the issues above have been corrected.

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