Release plan for v1.0

[HajimeKawahara]

Now, the first paper has been accepted by ApJS. So, we would start to prepare the version 1.0.

I think the main change from v0.8 includes

• more refactoring.
• add examples for new features, including a GP modeling of the correlated noise, layer-by-layer model, and so on.
• metal lines by @chonma0ctopus

[HajimeKawahara]

The release branch for v1.0.0 opened.

[HajimeKawahara]

I will check the following

• Does all the unit tests work?
• Does sample code in README.md work?
• Are explanations in API sufficient to understand?
• connection with Paper I?

[HajimeKawahara]

All the unit tests took 650sec 29 passed.

[HajimeKawahara]

• Check if all the tutorials work @astrostevanus checked. Thanks!