POC: Figure.subplot: Pythonic implemention for the subplot tagging

[seisman]

This PR provides a more Pythonic implementation for the -A option of subplot by splitting it into multiple parameters.

The full syntax is:

-A[autotag][+cdx[/dy]][+gfill][+j|Jrefpoint][+odx[/dy]][+ppen][+r|R][+s[[dx/dy][/shade]]][+v]

In PyGMT, the Pythonic parameters are:

• autotag: autotag [Previously it's called autolabel, but I feel it should be called autotag or tag]
• [+cdx[/dy]][+gfill][+ppen][+s[[dx/dy][/shade]]]: tag_box. Controls the box outline. It's a subset of the existing Box class, so we can directly pass a Box object!
• [+j|Jrefpoint][+odx[/dy]]: tag_position/tag_offset. Currently only +j is supported, but since it's very similar to the Position class, maybe we can make it accept a Position class?
• +r|R: tag_number_style
• +v: tag_orientation
• FONT_TAG: tag_font

It's a POC PR, and is not complete or final yet. Here is an example usage:

import pygmt
from pygmt.params import Box

fig = pygmt.Figure()
with fig.subplot(
    nrows=2, ncols=2, subsize=(5, 5), 
    autotag="(1)", 
    tag_box=Box(pen="1p,red", fill="lightblue"), 
    tag_position="TL", 
    tag_offset=(0.2, 0.2), 
    tag_orientation="vertical",
    tag_number_style="roman_upper",
    tag_font="12p,blue"
):
    for i in range(4):
        fig.basemap(region=[0, 10, 0, 10], projection="X?", panel=True)
fig.show()