[ADD] Change Hessian to purely PyTorch

[f-dangel]

Only review after #142

[f-dangel]

Ready for review.

[coveralls]

Pull Request Test Coverage Report for Build 11670381568

Details

• 11 of 16 (68.75%) changed or added relevant lines in 2 files are covered.
• 59 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.1%) to 88.61%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
curvlinops/utils.py	3	8	37.5%
<!--	Total:	11	16	68.75%	-->

Files with Coverage Reduction	New Missed Lines	%
curvlinops/_base.py	3	84.4%
curvlinops/examples/utils.py	10	31.25%
curvlinops/fisher.py	46	26.47%
<!--	Total:	59		-->

Totals
Change from base Build 11667562849:	-1.1%
Covered Lines:	1517
Relevant Lines:	1712

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💛 - Coveralls