Combine duplicate referential footnotes

[edelarua]

In many cases, the same referential footnote is used in multiple places. It would be a lot neater and easier to read if each unique referential footnote was only listed once and ref index numbers were instead repeated within tables.

Example (taken from test-header-footer.R):

Title says Whaaaat
Oh, ok.

——————————————————————————————————————————————————
           A: Drug X   B: Placebo   C: Combination
——————————————————————————————————————————————————
row1 {1}       5           5              5       
row2       1, 2 {2}     1, 2 {3}       1, 2 {4}   
——————————————————————————————————————————————————

{1} - row 1 rfn
{2} - row 2 cfn
{3} - row 2 cfn
{4} - row 2 cfn
——————————————————————————————————————————————————

ha HA! Footer!

changes to:

Title says Whaaaat
Oh, ok.

——————————————————————————————————————————————————
           A: Drug X   B: Placebo   C: Combination
——————————————————————————————————————————————————
row1 {1}       5           5              5       
row2       1, 2 {2}     1, 2 {2}       1, 2 {2}   
——————————————————————————————————————————————————

{1} - row 1 rfn
{2} - row 2 cfn
——————————————————————————————————————————————————

ha HA! Footer!

[gmbecker]

This is already done for non-number symbols, which are more appropriate for footnotes we expect to be repeated anyway. Doing so for automatically-numbered footnotes will significantly complicate things, I think.