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We mapped the Galactic young massive star cluster Westerlund 2 (Wd2) with theintegral field spectrograph MUSE (spatial resolution: 0.2arcsec/px, spectralresolution: $\Delta \lambda$ = 1.25A, wavelength range 4600-9350A) mounted onthe VLT, as part of an on-going study to measure the stellar and gas kinematicsof the cluster region. In this paper we present the fully reduced dataset andintroduce our new Python package "MUSEpack", which we developed to measurestellar radial velocities with an absolute precision of 1-2km/s without thenecessity of a spectral template library. This novel method uses thetwo-dimensional spectra and an atomic transition line library to createtemplates around strong absorption lines for each individual star. The coderuns fully automatically on multi-core machines, which makes it possible toefficiently determine stellar radial velocities of a large number of stars withthe necessary precision to measure the velocity dispersion of young starclusters. MUSEpack also provides an enhanced method for removing telluric linesin crowded fields without sky exposures and a Python wrapper for ESO's datareduction pipeline. We observed Wd2 with a total of 11 short and 5 longexposures to cover the bright nebular emission and OB stars, as well as thefainter pre-main sequence stars down to ~1M$_\odot$. The survey covers an areaof ~11arcmin$^2$ (15.8pc$^2$). In total, we extracted 1,725 stellar spectrawith a mean S/N>5 per pixel. A typical radial velocity (RV) uncertainty of4.78km/s, 2.92km/s, and 1.1km/s is reached for stars with a mean S/N>10,S/N>20, S/N>50 per pixel, respectively. Depending on the number of spectrallines used to measure the RVs, it is possible to reach RV accuracies of0.9km/s, 1.3km/s, and 2.2km/s with $\geq5$, 3-4, and 1-2 spectral lines,respectively. The combined statistical uncertainty on the radial velocitymeasurements is 1.10km/s.

The young massive star cluster Westerlund 2 observed with MUSE. II. MUSEpack -- a Python package to analyze the kinematics of young star clusters