Figure.solar: Support terminator datetime with timezone

[seisman]

The solar module can accept datetime with a timezone offset with a syntax like -Td+d2010-01-01T00:00:00+z08:00, but currently Figure.solar doesn't support it.

This PR adds the support for timezone.

Internally, GMT's solar module uses the C atoi function to parse the timezone after +z, so "08:00" -> 8, "-05:00"->-5, and fractional offsets are cast to integers, e.g. "08:30"->8, "-05:30"->-5.

Preview: https://pygmt-dev--4112.org.readthedocs.build/en/4112/api/generated/pygmt.Figure.solar.html

Address #4110.

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
added	pygmt/tests/baseline/test_solar_terminator_datetime_timezone.png

Image diff(s)

Added images

• test_solar_terminator_datetime_timezone.png

Modified images

Path	Old	New

Report last updated at commit caba47b

[yvonnefroehlich]

Maybe we can mention that it is the lower integer, e.g. 8.5 -> 8, not 9.

Suggested change

	Set the date and time for the terminator calculation. It can be provided as a
	string or any datetime-like object recognized by :func:`pandas.to_datetime`. The
	time can be specified in UTC or using a UTC offset. The offset must be an
	integer number of hours (e.g., -8 or +5); fractional hours (e.g., -8.5 or +5.5)
	are cast to integer. [Default is the current UTC date and time].
	Set the date and time for the terminator calculation. It can be provided as a
	string or any datetime-like object recognized by :func:`pandas.to_datetime`. The
	time can be specified in UTC or using a UTC offset. The offset must be an
	integer number of hours (e.g., -8 or +5); fractional hours (e.g., -8.5 or +5.5)
	are cast to the lower integer. [Default is the current UTC date and time].

[seisman]

I've changed it to "fractional hours (e.g., -8.5 or +5.5) are truncated towards zero (e.g., -8.5 becomes -8 and +5.5 becomes +5)."