Add the Box class for specifying the box of GMT embellishments

[seisman]

This PR implements the Box class that can be used in modules like coast/image/legend/colorbar/....

The full GMT CLI syntax is (https://docs.generic-mapping-tools.org/6.6/colorbar.html#f):

-F[+cclearances][+gfill][+i[[gap/]pen]][+p[pen]][+r[radius]][+s[[dx/dy/][shade]]]

The upstream long-form names are available at:

https://github.com/GenericMappingTools/gmt/blob/7026382558e11f89e2238bb345e1d0af325c12aa/src/longopt/psscale_inc.h#L35

• +c: clearance
• +f: fill
• +i: inner
• +p: pen
• +r: radius
• +s: shade

The Box class follows the upstream long-form names, but use inner_gap/inner_pen for +i and shade_offset/shade_fill for +s.

Preview:

• https://pygmt-dev--3995.org.readthedocs.build/en/3995/api/index.html#class-style-parameters
• https://pygmt-dev--3995.org.readthedocs.build/en/3995/api/generated/pygmt.params.Box.html

[seisman]

@GenericMappingTools/pygmt-maintainers Please give this PR a high priority, since many PRs are waiting for it.

[weiji14]

Looks good. Just two suggestions, one on ordering the docs, another one on making class BaseParam and AbstractBaseClass.

[weiji14]

Probably should make this an AbstractBaseClass (was debating between ABC or a Protocol).

Suggested change

	class BaseParam:
	from abc import ABC, abstractmethod, abstractproperty
	class BaseParam(ABC):

[seisman]

I've applied the suggestions but also made some changes in c6bd16f. The main changes are:

• abstractproperty is deprecated https://docs.python.org/3/library/abc.html#abc.abstractproperty. Now use @property and @abstractmethod.
• @abstractmethod means subclasses must implement the method/property, so the NotImplementedError is not needed and can be removed.
• The _validate method is meant to be optional, so we can't use @abstractmethod. Then ruff reports the B027 error, and it's ignored by adding the noqa flag.