Figure.legend: Support passing a StringIO object as the legend specification

[seisman]

Description of proposed changes

This PR adds StringIO support to Figure.legend, so that we can pass a StringIO object containing the legend specification to Figure.legend.

Need to wait for #3326.

Closes#571 and #576.

Example

importioimportpygmtspec=io.StringIO(
"""G -0.1iH 24p,Times-Roman My Map LegendD 0.2i 1pN 2V 0 1pS 0.1i c 0.15i p300/12 0.25p 0.3i This circle is hachuredS 0.1i e 0.15i yellow 0.25p 0.3i This ellipse is yellowS 0.1i w 0.15i green 0.25p 0.3i This wedge is greenS 0.1i f 0.25i blue 0.25p 0.3i This is a faultS 0.1i - 0.15i - 0.25p,- 0.3i A contourS 0.1i v 0.25i magenta 0.5p 0.3i This is a vectorS 0.1i i 0.15i cyan 0.25p 0.3i This triangle is boringD 0.2i 1pV 0 1pN 1M 5 5 600+u+fG 0.05iI @SOEST_block4.png 3i CTG 0.05iG 0.05iL 9p,Times-Roman R Smith et al., @%5%J. Geophys. Res., 99@%%, 2000G 0.1iT Let us just try some simple text that can go on a few lines.T There is no easy way to predetermine how many lines may be requiredT so we may have to adjust the height to get the right size box.""")
fig=pygmt.Figure()
fig.legend(spec, region=[0, 10, 0, 10], projection="M6i", position="x0.5i/0.5i+w5i+jBL+l1.2")
fig.show()

[weiji14]

🚀 Nice to close another >4 year old issue at #571!