make normal conjugate gradient available

[samwaseda]

Currently only ricQN is available (which is the new optimiser with the up-to-date CG parameters), but since this feature is not well tested, it's probably safer to offer the conventional CG as well (which is linQN)

[coveralls]

Pull Request Test Coverage Report for Build 500686398

• 10 of 13 (76.92%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.009%) to 64.67%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyiron_atomistics/sphinx/base.py	10	13	76.92%

Files with Coverage Reduction	New Missed Lines	%
pyiron_atomistics/sphinx/base.py	1	81.67%

Totals
Change from base Build 499117394:	0.009%
Covered Lines:	10558
Relevant Lines:	16326

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💛 - Coveralls