src: allow for year = 0; BC --> BCE; facilitate Astronomical and (B)CE timeline

[evgaster]

A previous fix for issue 1538 incorrectly rules out year = 0; Removed that restriction.
Changed the BC indication to the preferred BCE.
Facilitate both the Astronomical and the (B)CE timeline.
Make the label for the DateMenu consistent.
Improve Date and Time documentation.
Replaced the code that converts between Julian en Gregorian dates by
functions from the Standards Of Fundamental Astronomy (SOFA) Libraries.

Signed-off-by: Ed@vanGasteren.net

[c3d]

Thanks for the work, @evgaster. I cannot take this PR as is without first ensuring that we can respect the license. At first glance, it looks like I need to be a bit careful.

[evgaster]

Wanted to discuss "the license" "issue" with you.
I assume you read the copyright / license comment block at the very end of datetime.cc.

Because of that I prefer to keep the needed parts of the SOFA library intact in separate unchanged source files. But that meant changes to the Makefile. As an intermediate step I inserted the routines in datetime.*.

My interpretation of the "copyright / license comment block" is that if we move to using "separate unchanged source files" then there is acknowledgement to do and we're done.

[evgaster]

At the moment it is "derived work" and we should rename the functions.
I prefer to move to "separate unchanged source files" to get rid of "derived work" and the conditions that apply to that.

[evgaster]

@c3d what changes do I need to the Makefile to have the compile and build pickup additional files:

• cal2jd.c
• jd2cal.c

which depend on:

• sofa.h
• sofam.h

I hoped only the below change would do the trick (but it doesn't):

C_SOURCES +=                            \
        src/cal2jd.c                    \
        src/jd2cal.c

Then I can remove the sofa stuff I inserted in datatime.* and only insert #includes for the sofa*.h in datatime.cc. Then we comply with the sofa license.

[evgaster]

@c3d see this commit concerning your "I need to be a bit careful".

Please help me with the Makefile issue.

Further I don't understand when to use '+' or not in constructs like:

            time_part = +fraction::make(integer::make(time),
                                        +integer::make(scale_from_cc));

copied this from an other part ... but don't fully understand.

Then there is:

        // How on earth do I get the real or double value out of jval?

would appreciate your help with that.

I will look into (twice):

            // ToDo: handle errors.

but again, would appreciate your help with that.