feat(ring_theory/hahn_series): summable families of Hahn series (#6997)
feat(ring_theory/hahn_series): summable families of Hahn series (#6997)

Defines `hahn_series.summable_family`
Defines the formal sum `hahn_series.summable_family.hsum` and its linear map version, `hahn_series.summable_family.lsum`




Co-authored-by: Aaron Anderson <6a708ae528ec3e8bbcd87cbd53c0d4f4bab63d64@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib