Xtrack version 0.67.0

Changes:

• Introduce Environment for rapid manipulation of elements, lines and expressions in lattice design.
• Change algorithm for periodic_symmetric mode in Twiss.
• Better plotting features TwissTable.plot()
• Introduce SurveyTable.plot()
• Introduce gzip mode in json. Json files are now indented by default.
• Fix for time-dependent vars

Full Changelog: v0.66.1...v0.67.0