stats::aggregate now renamed argument formula to x

[xtYao]

Description:

• Renaming the "formula" argument in "stats::aggregate" to "x"
• Latest R base no longer has "formula" as the first argument of "stats::aggregate" function, causing gTrack to fail when "vaggregate" function is called. See https://rdrr.io/r/stats/aggregate.html

Changes:

• Renaming the "formula" argument in "stats::aggregate" to "x"

Impact:

• gTrack would fail without this change if vaggregate is triggered anywhere along the call stack

[xtYao]

Somehow this is not the case in the unit tests. Maybe a diff in R versions used.