POC: Add a LegendSpec class for building legend specifications

[seisman]

Implement a LegendSpec class to create legend specification file in a more readable way (initial idea from #1513 (comment)).

This is still POC. So only three legend records are implemented (xref: https://docs.generic-mapping-tools.org/6.7/legend.html#legend-codes).

import pygmt
from pygmt.params import LegendSpec

# Build a legend specification 
spec = LegendSpec()
spec.add_header("My Legend")
spec.add_symbol(symbol="c", size=0.2, label="Circle", pen="1p,black")
spec.add_symbol(symbol="s", size=0.3, label="Square", fill="blue", pen="2p,red", dx1=0.5, dx2=2.0)
spec.add_line(length=1.0, pen="1p,blue", label="Line 1")
spec.add_line(length=1.0, pen="1p,red,-", label="Line 2", dx1=0.5, dx2=2.0)

print(spec)

fig = pygmt.Figure()
fig.basemap(region=[0, 10, 0, 10], projection="X10c/10c", frame=True)
fig.legend(spec=spec)
fig.show()

The output is:

H - My Legend
S - c 0.2 - 1p,black - Circle
S 0.5 s 0.3 blue 2p,red 2.0 Square
S - - 1.0 - 1p,blue - Line 1
S 0.5 - 1.0 - 1p,red,- 2.0 Line 2