Fix backend filename if the user changes the h5 filename

[zihaoxu98]

If someone initialize the context by appletree.Context.from_backend(file_name), the backend_h5 will be from the instruct stored in the backend file, not the file name of the backend. It's buggy if someone changes the backend file name after a fitting. This PR forces the backend_h5 to be the correct file name

[coveralls]

Pull Request Test Coverage Report for Build 10707213964

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 85.196%

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Totals
Change from base Build 10707206869:	0.006%
Covered Lines:	2440
Relevant Lines:	2864

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💛 - Coveralls