Minor updates for the support of functions
Minor updates for the support of functions

- correction of suppssrNEW
- New sections "Finitely supported functions (extension)" and "Group sum operation (extension)" in AV's Mathbox (and rearranging theorems accordingly
- Adding new theorems using finSupp instead of "( . supp . ) e. Fin"

metamath/set.mm,metamath/set.mm,metamath/set.mm