Made cosmetic changes to PopDiffSequenceData.Rmd

[zkamvar]

I made some changes to @mmlopezu's vignette to make things a bit more readable. This should only be merged with @mmlopezu's approval.

List of changes:

• removed extraneous yaml
• headers standardized (# instead of ====)
• add newline before code chunks
• code linted (spaces around binary operators and after commas)
• setup separated into three parts
• strata set up at beginning of vignette

[zkamvar]

@mmlopezu, do you have any concerns with this vignette?

[mmlopezu]

@zkamvar Thanks for the edits! I will add the conclusion tonight.

[zkamvar]

Thanks for the edits! I will add the conclusion tonight.

No problem :smile: