Xtrack version 0.94.0

Changes:

• Correctly handle positions RBends when computing drift lengths with specified s positions.
• Support expressions in at arguments of Environment.place().
• Environment.elements, Environment.vars, Environment.lines, Environment.particles implemented by dedicated container classes. It is enforced that names in the containers cannot overlap.
• Introduce mode="compose" for Line
• Implement Environment.elements.remove() that takes care of keeping consistency at the xdeps level.
• Handle BeamElement.extra and BeamElement.prototype throughout the environment life cycle (including copy(), to_dict(), from_dict()).
• Change behavior of line.replicate(...) and line.clone(...): they take suffix instead of name and return a line instead of storing one in line.env.lines.
• Fix in BaseElement.to_dict()
• Adapt examples and tests to use recommended API (env.elements instead of element_dict/element_refs, env.ref[...] instead of env.vars[...] and env.element_ref[...]).

Full Changelog: v0.93.2...v0.94.0