Fixing GOSPA metric to work with ParticleState

[ollie-digi2al]

There was an issue converting a list of ParticleState objects into a numpy array. This pull request fixes this by replacing
np.array(measured_states)[meas_mask]

with

meas_points = np.empty(len(measured_states), dtype="O")
meas_points[:] = measured_states
meas_points = meas_points[meas_mask]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.49%. Comparing base (10e26e0) to head (2503d26).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #967      +/-   ##
==========================================
- Coverage   93.52%   93.49%   -0.04%     
==========================================
  Files         201      201              
  Lines       12835    12837       +2     
  Branches     2629     2629              
==========================================
- Hits        12004    12002       -2     
- Misses        587      589       +2     
- Partials      244      246       +2     

Flag	Coverage Δ
integration	66.26% <0.00%> (-0.02%)	⬇️
unittests	89.09% <100.00%> (-0.03%)	⬇️

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