Add in-situ diagnostic for spin

This PR extends the beam in-situ diagnostic to include the first and second moment of spin in each direction if spin tracking is enabled. The following python script can read and plot the in-situ spin diagnostics:

import sys
sys.path.insert(1, "../hipace/tools/")
import read_insitu_diagnostics as diag
import matplotlib.pyplot as plt
import numpy as np

path = "/scratch/project/alex/debug/diags/new/spin_track_19/insitu/reduced_beam*"

all_data = diag.read_file(path)

time = all_data["time"]
sx_mean = all_data["average"]["[sx]"]
sy_mean = all_data["average"]["[sy]"]
sz_mean = all_data["average"]["[sz]"]
sx_std = np.sqrt(np.maximum(all_data["average"]["[sx^2]"] - all_data["average"]["[sx]"]**2, 0))
sy_std = np.sqrt(np.maximum(all_data["average"]["[sy^2]"] - all_data["average"]["[sy]"]**2, 0))
sz_std = np.sqrt(np.maximum(all_data["average"]["[sz^2]"] - all_data["average"]["[sz]"]**2, 0))
pol = np.sqrt(sx_mean**2 + sy_mean**2 + sz_mean**2)

fig, ax = plt.subplots(4, 1, dpi=150, figsize=(8, 11))
ax[0].plot(time, sx_std, label="HiPACE++ std sx")
ax[0].plot(time, sy_std, label="HiPACE++ std sy")
ax[0].plot(time, sz_std, label="HiPACE++ std sz")
ax[0].legend()
ax[0].axes.xaxis.set_ticklabels([])

ax[1].plot(time, 1-sx_mean, label="HiPACE++ 1-mean sx")
ax[1].legend()
ax[1].axes.xaxis.set_ticklabels([])

ax[2].plot(time, sy_mean, label="HiPACE++ mean sy")
ax[2].plot(time, sz_mean, label="HiPACE++ mean sz")
ax[2].legend()
ax[2].axes.xaxis.set_ticklabels([])

ax[3].plot(time, 1-pol, label="HiPACE++ 1-spin polarisation")
ax[3].legend()
ax[3].set_xlabel("time in s")

plt.tight_layout()

[ ] Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
[ ] Tested (describe the tests in the PR description)
[ ] Runs on GPU (basic: the code compiles and run well with the new module)
[ ] Contains an automated test (checksum and/or comparison with theory)
[ ] Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
[ ] Constified (All that can be const is const)
[ ] Code is clean (no unwanted comments, )
[ ] Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
[ ] Proper label and GitHub project, if applicable

Hi-PACE / hipace

Add in-situ diagnostic for spin #1086