Make "piecewise" the default pdf interpolation

[hammannr]

This PR makes "piecewise" the default pdf interpolation method. Before, the default was defined in blueice to "linear", which is only in certain limits compatible with the BlueiceDataGenerator. The BlueiceDataGenerator is now only assigned if all sources have piecewise interpolation -- in the future we can add a suitable generator also for the linear interpolation case.

[coveralls]

Pull Request Test Coverage Report for Build 8087518258

Details

• 12 of 13 (92.31%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 90.738%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
alea/models/blueice_extended_model.py	8	9	88.89%

Totals
Change from base Build 8082027034:	0.007%
Covered Lines:	1450
Relevant Lines:	1598

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💛 - Coveralls

[kdund]

Looks good for me-- any way to test?

[hammannr]

@kdund you could add pdf_interpolation_method="linear" in one of the example configs and try to load the model -- it should complain in the initialization step that it doesn't like it. If you add pdf_interpolation_method="piecewise" or nothing at all it should work fine.

[kdund]

Checked, and works, thanks @hammannr

[dachengx]

Hey @hammannr . Maybe you could try to squash and merge next time. Then it is easier to track the change from a PR in the GitHub history.