Need feedbacks on the Jupyter lab for trying GMT and its wrappers online

This repository sets up a single Jupyter lab environment, with the latest/stable GMT, PyGMT, and GMT.jl installed. The idea and some contents of this repository are from our older projects, [gmt6demo](https://github.com/GenericMappingTools/gmt6demo), [try-gmt-python](https://github.com/GenericMappingTools/try-gmt-python), and [foss4g2019oceania](https://github.com/GenericMappingTools/foss4g2019oceania).

Anyone can run GMT commands (i.e., in bash), PyGMT, and GMT.jl in this online Jupyter lab, with no need to install anything locally.

I believe this Jupyter lab would be useful for both new and old GMT users who want to try GMT and its wrappers before spending some time setting up a local environment. It also provides a uniform environment for future GMT workshops and short courses, so that we don't have to spend hours solving the GMT installation problems (I'm not sure about its performance if dozens of peoples use it simultaneously).

The Jupyter lab is freely hosted by [Pangeo Binder](https://binder.pangeo.io/). You can try it by clicking the badge below:

[](https://binder.pangeo.io/v2/gh/GenericMappingTools/try-gmt/master?urlpath=lab/tree/landing-page.ipynb)

it usually takes <30 s to launch the Jupyter lab, the you can try the three demos for bash, Python, and Julia, respectively.

## Future Plans

Here are some of my thoughts to improve it:

- [x] Cache the low-resolution (>06m) earth relief data so that users don't have to wait for downloading them [**Done in #4**]
- [ ] Install a few extra Python packages (e.g., scipy, matplotlib) and Julia packages to make it more suitable for practical plots
- [ ] Create a `dev` branch and set up another Jupyter lab environment with the master/unstable/developing GMT, PyGMT, and GMT.jl installed.
- [x] Host it on [Google's colab servers](https://colab.research.google.com/) (also free), which seem faster.
- [x] Post it on the GMT forum to increase its visibility (https://forum.generic-mapping-tools.org/t/try-gmt-pygmt-gmt-jl-online-at-pangeo-binder/561)
- [x] Add a "Try it online!" menu on the GMT homepage to increase its visibility (https://github.com/GenericMappingTools/website/pull/71)
- [ ] Add "Try it online!" badges to the README file of GMT, PyGMT and GMT.jl repositories?

Any thoughts and suggestions? @GenericMappingTools/core @GenericMappingTools/gmt-contributors @GenericMappingTools/python @GenericMappingTools/python-contributors