DGE helper function - p-value formatting

[dtm2451]

Initializes a helper function that can transform a vector of p-values into various alternative labeling formats. It can also be used for simply rounding without that rounding causing, e.g., a 0.05 to then show as 0.0500. Summary formats include the below, but are quite tweak-able both in terms of style and the numeric cutoffs.

• NS, *, **, ***
• NS, padj = <pval> (which can be re-styled to NS, <pval>, or NS, p:<pval>)
• NS, padj = <pval>, padj < 0.001, padj < 0.0001 (which can be re-styled to NS, <pval>, <0.001, <0.0001)

Among others -- fully documented in the code -- this 'padj' element is adjustable to 'p', something else, or removed altogether via the value_style_p_string input.

[dtm2451]

Some notes:

• The current design inputs/outputs a simple vector of p-values, not data.frame including them
  • Rationale: This allows for flexibility in usage or column naming for how the output might get added to a data.frame afterwards.
  • Down-side: It precludes trimming inside the function of data.frame rows with non-significant p-values, because the full data.frame is not provided.