Set initial magnetic moments

[samwaseda]

This allow the user to set the magnetic moments via single value or dict:

Example I

structure = pr.create.structure.bulk('Ni', cubic=True)
structure[-1] = 'Fe'
v_dict = {'Fe': 1, 'Ni': 2}
structure.set_initial_magnetic_moments(v_dict)
print(structure.get_initial_magnetic_moments())

Output: array([2, 2, 2, 1])

Example II

structure = pr.create.structure.bulk('Ni', cubic=True)
structure[-1] = 'Fe'
structure.set_initial_magnetic_moments(1)
structure.get_initial_magnetic_moments()

Output array([1, 1, 1, 1])

Moreover, I saw that the default behaviour was setting the system non-magnetic, i.e. regardless of what was set for the magnetic moments, structure.set_initial_magnetic_moments() would make the structure non-magnetic. This is now disabled, i.e. if the user wants to make it non-magnetic, they are required to explicitly set structure.set_initial_magnetic_moments(None)

[coveralls]

Pull Request Test Coverage Report for Build 3639206527

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 68.674%

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Totals
Change from base Build 3622625190:	0.01%
Covered Lines:	12121
Relevant Lines:	17650

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💛 - Coveralls

[pmrv]

Some nits, but generally ok with me.

[ligerzero-ai]

All done - let's get this merged.