Merge branch 'develop' into 'master'

Merge pull request e2nIEE#1164 from e2nIEE/develop

Release pandapower 2.6.0

Merge pull request e2nIEE#1092 from SimonRubenDrauz/release_2.5.0

Release 2.5.0

Merge pull request e2nIEE#906 from wangzhenassd/develop

Fix py35 estimation test fail

fix unbalanced element docs

Merge pull request e2nIEE#863 from dlohmeier/chefPony-feature/createM…

…ultipleElements

Chef pony feature/create multiple elements

Merge branch 'develop' of https://github.com/e2nIEE/pandapower into d…

…evelop

Merge pull request e2nIEE#635 from FlorianShepherd/develop

pandapower 2.2.1

Merge pull request e2nIEE#614 from FlorianShepherd/develop

pandapower version tag, copyright etc for v 2.2.0

release pandapower 2.0.1