Fix failed saving strategy "better", update docs, and release v0.7

[WenjieDu]

What does this PR do?

1. fix Model saving bug #464;
2. update docs and fix Docs building failed because of Unicode #471;
3. release v0.7;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 10026017637

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.035%

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Totals
Change from base Build 10019927876:	0.0%
Covered Lines:	10580
Relevant Lines:	12590

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💛 - Coveralls