Include stable multi-period version

[jokochems]

Problem description

The oeomof.solph multi-period feature has been under development for quite some time. In the mean time, oemof.solph has been restructured quite rigorously. While updating to the latest structure changes, it does make sense to include a stable version in the dependencies.

Suggested fix

• Add a copy of environments.yaml
• Use latest stable commit hash to install dependencies.