Xtrack version 0.70.2

Changes:

• Add support for refer=exit in the definition of lines
• Introduce xt.load_madx_lattice
• Introduce Line.vars.load_madx
• Introduce TwissTable.get_R_matrix_table()
• Fixes and enhancements in TwissTable.plot(...)

Full Changelog: v0.70.1...v0.70.2