Set up bracketing for spatial solvers.

[Vilin97]

This is in preparation for the spatial SSA RSSACRDirect.

[isaacsas]

Would it make sense to have RSSA/RSSACR store the high/low values together in general? i.e. is it likely that whenever we need high we may shortly thereafter need low, and so should just return both? I wonder if that might give a perf bump by storing them together in memory.

[Vilin97]

I do like the little handy LowHigh struct. It makes RSSA code look similar to normal code, which is good for maintainability. I'm not sure about performance, that would need to be measured. My instinct is that we would not win much performance by switching non-spatial RSSA-based SSAs to LowHigh.

[coveralls]

Pull Request Test Coverage Report for Build 6138963328

• 24 of 29 (82.76%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 89.84%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/spatial/bracketing.jl	20	25	80.0%

Totals
Change from base Build 6133295964:	-0.05%
Covered Lines:	2299
Relevant Lines:	2559

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💛 - Coveralls