add timing of backwards pass

[pat-schmitt]

Time the cpu time of cost_function.backward(), where gradients are calculated.

[coveralls]

Pull Request Test Coverage Report for Build 16263010284

Details

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 65.477%

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Files with Coverage Reduction	New Missed Lines	%
agile1d/sandbox/create_glaciers_with_measurements.py	1	98.65%

Totals
Change from base Build 6048884072:	0.04%
Covered Lines:	2274
Relevant Lines:	3473

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💛 - Coveralls