Merge pull request pybamm-team#931 from pybamm-team/develop

Version 0.2.1 develop -> master

Merge pull request pybamm-team#856 from pybamm-team/issue-820-release

Issue 820 release

Merge pull request pybamm-team#627 from pybamm-team/issue-623-soc_enh…

…ancement

Issue 623 soc enhancement