Release v0.3.0

[seisman]

We will make the v0.3.0 release when PR https://github.com/gmt-china/GMT_docs/pull/364 is ready to merge.

• [x] Document all the important changes in Changelog.md
• [x] Set the release date in Changelog.md
• [x] Set the current release version and date in README.md

[seisman]

@core-man @CovMat 你们俩有谁想 release v0.3.0？

[core-man]

@core-man @CovMat 你们俩有谁想 release v0.3.0？

I can help if @CovMat is not available, but I think he should do this since he contribute most of the current dataset.

[seisman]

@core-man You can make the release if you have time.

[core-man]

@core-man You can make the release if you have time.

Okay~