Xpart

**New feature:**
- Introduced flags remove_underscored, remove_unused_space, remove_redundant_variables, keep_rng_state, keep_rng_state, compact in Particles.to_dict and Particles.to_pandas

**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)