v0.6.8

EnergyModelsBase v0.6.8

Diff since v0.6.7

Merged pull requests:

• Added potential for negative emission technologies (#22) (@JulStraus)
• Added a contribution section to the documentation (#23) (@JulStraus)
• Docs/fix typos (#24) (@JulStraus)
• Removed \texttt in docstrings (#25) (@JulStraus)

Closed issues:

• Consider dropping texttt for variables (#20)