Xpart

**Fix:**
- Solves issue with unallocated slots in Particles.__init__, Particles.to_dict and Particles.from_dict.

**New feature:**
- Energy variables in Particles (delta, psigma, rvv, rpp) are now linked using xobjects.LinkedArray (e.g. part.delta[3] = 0.1 automatically updates also psigma, rvv, cpp)

**New features:**
- LocalParticle_add_to_energy can add to p_z only or to all momentum components (needed for synchrotron radiation)
- Added LocalParticle_generate_random_double and LocalParticle_generate_random_double to random number generator (needed for synchrotron radiation)

**Fix:**
- Include header files in pypi package

**New features:**
- Generation of halo and pencil distributions.
- Particles.to_dict creates by default a copy of the data on CPU.
- Introduced Particles.merge and Particles.filter methods.
- Introduced Particles.to_pandas and Particles.from_pandas methods.
- Introduced examples on how to save/load particles from/to file.
- build_particles and generate_matched_gaussian_bunch can find automatically the closed orbit and the one-turn matrix from a xtrack.Tracker object.
- Introduced explicit modes (set, shift, normalized_transverse) as input of the build_particles function.
- generate_longitudinal can use a xtrack.Tracker object to find the required machine parameters (RF voltage, harmonics, phases, momentum compaction factor).

**Fix**
- In the Particles.from_dict method all energy variables are taken from the dictionary (before they were all recomputed from delta)

**Change in the Xsuite package structure:**
- The xline package is removed
- xline.Line becomes xtrack.Line
- instead of xline.Drift, xline.Multipole etc. use directly xt.Drift, xt.Multipole
- The Tracker objects are automatically updated when the line is modified (no need to regenerate the tracker)
- There is only one Particles class within Xsuite
- xline.Partices and xtrack.Particles to be replaced by xpart.Particles
- The reference python implementation of the main elements, which used to be in xline, is moved to a small package called ducktrack, installed together with xtrack, to be used only for testing purposes (not part of normal tracking workflow)

**New features**
- Added build_particles function allowing to build a particles object from a reference particles plus arrays with geometric or normalized coordinates
- Added tools to compute and symplectify one-turn matrices (moved from pymask)
- Random number generator moved from xtrack togehter with the Particle class
- Ported PyHEADTAIL interface from xtrack