Fix various Calc date conversions (bug#36822)

Fix various Calc date conversions (bug#36822)

* lisp/calc/calc-forms.el (math-absolute-from-gregorian-dt):
Rewrite in a way that I understand, and that actually seems to work.
(math-absolute-from-julian-dt): Use Julian, not Gregorian, leap year
rules for counting days within a year.
(math-julian-date-beginning, math-julian-date-beginning-int):
Change constants to be consistent with their doc strings and the code:
use Rata Die epoch at Dec 31, 1 BC Gregorian proleptic, not Julian.
* doc/misc/calc.texi (Date Forms): Correct difference between Julian
Day and Rata Die.
* test/lisp/calc/calc-tests.el (calc-test-calendar): New test.