Xtrack

**New feature:**
- Possibility to skip kernel compilation when creating tracker (to access the source without compiling)

**Changes:**
- Add copyright statement to all files
- Add license file
- Add metadata in setup.py

**Change:**
- Definition of longitudinal coordinate zeta changed from zeta = s beta/beta0 - beta c t to zeta = s - beta0 c t.

**New features:**
- io_buffer and internal records for data logging inside beam elements
- Logging of emitted photons in Multipole
- possibility to unfreeze a line (invalidating the associated tracker)
- Fix in synchrotron radiation

**Changes:**
- Introduced FirstOrderTaylorMap.
- Tracker can start and stop at any element of the line both in collective and non-collective mode.
- LossLocationRefinement by default allows backtracking only through a restricted list of element types.
- Fix initialisation fo random number generation by Tracker in collective mode.
- Preserve order of elements at the same s coordinate when imported from MAD-X.
- Import polygon aperture from MAD-X.
- Import deferred expressions from MAD-X in Wire element.
- Moved generation of Line from sixtrack input to sixtracktools package.

**Changes:**
- Adapt to Xdeps v0.0.5 API
- Removed default to zero for variables used in Xdeps expressions (default is disabled after MAD-X import)