POC: Implement the general BaseParams class for any class-like parameters

[seisman]

This PR contains some experimental codes towards the Pythonic interface discussed in #1082. Generally speaking, this PR implements the base class BaseParams and any parameters that expect a class-like argument can inherit from it (a similar work is done in PR #1239).

In this PR, a Box class is implemented showing how the BaseParams can be used. The Box class can be passed to the box parameter in multiple modules (image, logo, basemap, coast, inset, legend and colorbar). The full syntax of the box parameter is (https://docs.generic-mapping-tools.org/dev/image.html#f):

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

Here is an example showing how the Box class can be used:

import pygmt
from pygmt import params

fig = pygmt.Figure()
for i, box in enumerate(
    [
        params.Box(fill="red@20"),
        params.Box(clearance=(0.2, 0.2), fill="red@20", pen="blue"),
        params.Box(clearance=(0.2, 0.2), pen="blue", radius=True),
        params.Box(clearance=(0.1, 0.2, 0.3, 0.4), pen="blue", radius="10p"),
        params.Box(
            clearance=0.2, pen="blue", radius="10p", shading=("5p", "5p", "lightred")
        ),
        params.Box(clearance=0.2, pen="blue", innerborder=("2p", "1p,red")),
    ]
):
    print("###", i)
    print(repr(box))
    print(str(box))
    print()
    fig.logo(position="x0/0+w3c", box=box)
    fig.shift_origin(xshift=4)
fig.show()

The console outputs are:

### 0
Box(fill='red@20')
+gred@20

### 1
Box(clearance=(0.2, 0.2), fill='red@20', pen='blue')
+c0.2/0.2+gred@20+pblue

### 2
Box(clearance=(0.2, 0.2), pen='blue', radius=True)
+c0.2/0.2+pblue+r

### 3
Box(clearance=(0.1, 0.2, 0.3, 0.4), pen='blue', radius='10p')
+c0.1/0.2/0.3/0.4+pblue+r10p

### 4
Box(clearance=0.2, pen='blue', radius='10p', shading=('5p', '5p', 'lightred'))
+c0.2+pblue+r10p+s5p/5p/lightred

### 5
Box(clearance=0.2, innerborder=('2p', '1p,red'), pen='blue')
+c0.2+i2p/1p,red+pblue

The generated image is:

Known limitations that should be addressed:

• innerborder=('2p', '1p,red') is not ideal. I expect to see innergap="2p", innerpen="1p,red" instead.
• shading=('5p', '5p', 'lightred') is not idea, shading_offset=("5p", "5p") and shading_fill="lightred" is better