Implement power shrinking

[daffidwilde]

The shrinking process is now done using a power law around the intervals for each parameter:

upper_after - lower_after == (upper_before - lower_before) * (shrinkage ** iteration)

The interval is centred about the "midpoint", i.e. the mean, still.

[coveralls]

Pull Request Test Coverage Report for Build 141

• 27 of 27 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 138:	0.0%
Covered Lines:	388
Relevant Lines:	388

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💛 - Coveralls