feat(algebra/big_operators): sum_ite (#1598)
feat(algebra/big_operators): sum_ite (#1598)

* feat(algebra/big_operators): sum_ite

rename the current `sum_ite` to `sum_ite_eq` and add a more general version

* Update src/algebra/big_operators.lean

Co-Authored-By: Johan Commelin <759412786bc533369b22377bf83fb9056c5b25b2@commelin.net>

leanprover-community/mathlib,leanprover-community/mathlib