💫 Update subglacial lake maps for AGU2021 Fall Meeting

Refresh Antarctic active subglacial lake maps using new data up to 20210715. Specifically, we are using subglacial lake outlines from commit weiji14/deepicedrain@9073678 in weiji14/deepicedrain#329. Main code change is to update to PyGMT v0.5.0 syntax, such as directly plotting geopandas.GeoDataFrame objects! Text label placements remain the same, and I've thrown in the .geojson to .gmt conversion script (requires deepicedrain kernel) at the end of the notebook. Also updated thumbnails on main README.md.

Author: weiji14

README.md

@@ -9,8 +9,8 @@ happening in New Orleans & Online everywhere from 13-17 December 2021.
 [![nbviewer](https://img.shields.io/badge/jupyter_notebooks-nbviewer-purple.svg?style=flat-square)](https://nbviewer.jupyter.org/github/weiji14/agu2021/blob/main/key_figure.ipynb)
 
 ![AGU2021 DeepIceDrain poster](https://user-images.githubusercontent.com/23487320/107176091-4b0dbd00-6a33-11eb-9eba-2d13959e43a8.png)
-![Siple Coast active subglacial lakes](https://user-images.githubusercontent.com/23487320/107176140-6aa4e580-6a33-11eb-8f30-1a7913788d6c.png)
-![Antarctic active subglacial lakes](https://user-images.githubusercontent.com/23487320/112776311-d7d01100-909b-11eb-98df-1c8fa106f34e.png)
+![Siple Coast active subglacial lakes](https://user-images.githubusercontent.com/23487320/145827669-6fe3581a-af92-41ea-ba9e-11256a5c09a5.png)
+![Antarctic active subglacial lakes](https://user-images.githubusercontent.com/23487320/145827635-89af64e5-e74e-4834-b793-b7332ff1291e.jpg)
 
 ## Quickstart
 

antarctic_subglacial_lakes_3031.gmt

@@ -1,9 +1,9 @@
--3	255/173/173	-2	167/89.5/73	L	
--2	167/89.5/73	-1	80/24/3	L	
--1	80/24/3	0	25/12/9	L	
-0	24/12/10	1	26/66/86	L	
-1	26/66/86	2	61/143/186.5	L	
-2	61/143/186.5	3	158/176/255	B	
+-3	255/173/173	-2	167/89.5/73	L
+-2	167/89.5/73	-1	80/24/3	L
+-1	80/24/3	0	25/12/9	L
+0	24/12/10	1	26/66/86	L
+1	26/66/86	2	61/143/186.5	L
+2	61/143/186.5	3	158/176/255	B
 B	255/173/173
 F	158/176/255
 N	white