Add the Frame/Axis class for setting frame and axes [Part 1]

[seisman]

This PR implements the Frame/Axis class for setting frame and axes attributes. It's a subset of PR #4016 because the -B option is complicated, and it will take a long time to finish #4016, which again will delay the paper submission.

Currently, only a small subset of the -B option is implemented in this PR, but it should already cover most use cases.

Here is a comparison table between the GMT CLI and the PyGMT versions:

GMT	PyGMT
-B	frame=True
-BWSEN	frame=Frame("WSEN") or frame=Frame(axes="WSEN")
-BWSEN+tMY TITLE	frame=Frame("WSEN", title="MY TITLE")
-Ba	frame=Axis(annnot=True)
-Bafg	frame=Axis(annot=True, tick=True, grid=True)
-Ba30f15g5	frame=Axis(annot=30, tick=15, grid=5)
-Ba30f15g5+lLABEL	frame=Axis(annot=30, tick=15, grid=5, label="LABEL")
-BWSEN+tTITLE -Bxa30f15g5+lXlabel -Bya4f2g1+lYlabel	frame=Frame("WSEN", title="TITLE", xaxis=Axis(annot=30, tick=15, grid=5, label="Ylabel"), yaxis=Axis(annot=4, tick=4, grid=1, label="Ylabel"))

This PR doens't break backward-compatibility, so frame="WSEN", frame="afg", frame="a30f15g5", frame=["WSEN+tTITLE", "xafg", "yafg"] are still supported.