Use of `.x` and `.` to refer resp. to original data and to groups (or subsetted data)

[moodymudskipper]

• [x] .x is the name of the first arg, we just make sure it's accessible in every argument
• [x] .x is the original data before subsetting with .i or .j,
• [x] .data is the data current data, considering the previous transformations that occured in ...
• [x] .subset is similar to what data.table calls .SD, it's the given group of subsetted data when grouping, it's the same as .data if we have no group.

[moodymudskipper]

.data and .subset are a bit long, and we changed .x into .X because it was conflictiing ,

think we should go for :

• .X is the name of the first arg, we just make sure it's accessible in every argument, it is the original data before subsetting with .i or .j,
• .D, like Data, is the current data, considering the previous transformations that occured in ..., same as .X initially
• .C, like Chunk, is similar to what data.table calls .SD, it's the given group of subsetted data when grouping, it's the same as .data if we have no group. Same as .D outside of grouping/along operations
• .N is nrow(.C)