Initialize context from backend

[zihaoxu98]

This PR adds a class method that allows context to be initialized from a backend file. The continue_fitting is also modified a bit for better experience.

If you have a backend from a fitting, and want to continue, now you just need to

tree = apt.Context.from_backend('./backend.h5')
tree.continue_fitting(iteration=10)

The original backend file will be appended to new walkers.

[dachengx]

Thanks @zihaoxu98 . Looks good. Please try to add pytest of continue_fitting and from_backend.

[coveralls]

Pull Request Test Coverage Report for Build 8583100372

Details

• 26 of 28 (92.86%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 85.47%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
appletree/context.py	25	27	92.59%

Totals
Change from base Build 8582131298:	0.3%
Covered Lines:	2300
Relevant Lines:	2691

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💛 - Coveralls