Xtrack

**Changes:**
- Fix in buffer check at tracker creation

**Changes:**
- Twiss can switch to 4d method to compute optics parameters in the absence of a stable longitudinal motion (RF off, frozen longitudinal coordinates)
- Twiss can handle a portion of a beam line
- Introduced TwissTable objects (for twiss calculation results).
- Introduce TwissTable.reverse(...) method to generate twiss table for the reversed beam line (e.g. b2 twiss from b4 twiss)
- Twiss results can be easily transferred to a pandas data frame.
- 4d beam correlation matrix (Sigma matrix) can be calculated from the twiss result TwissTable.get_betatron_sigmas(...)).
- Introducee Tracker.match method to find values for a set of accelerator knobs in order to match given targets in the twiss results.
- Introduced Tracker.survey method.
- Version number can be found in xtrack.__version__.

**Full Changelog**: https://github.com/xsuite/xtrack/compare/v0.19.4...v0.20.0

**Changes:**
- Update following method rename in xobjects
- Line.from_dict significantly faster following xobject optimisation

**Changes:**
- Handle local_particle_src in a way that is compatible with python multithreading

**Changes:**
- Rename CustomSetter to MultiSetter.
- Fix in mad_loader (import of crabcavity element)

**Changes:**
- Introduced CustomSetter class to simultaneously apply changes to a large number of element properties