Update paper to improve readability

[prmiles]

• Added paragraph(s) to explain big picture behind model calibration in light of uncertainty.
• Moved paragraphs around to explain motivation behind the package development earlier in the paper.

[coveralls]

Pull Request Test Coverage Report for Build 378

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.004%

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Totals
Change from base Build 371:	0.0%
Covered Lines:	2210
Relevant Lines:	2255

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💛 - Coveralls