Fix numpy DeprecationWarning in burn_in_by_sequential_geweke

[dweindl]

Fixes

test/sample/test_sample.py: 542 warnings
  pyPESTO/pypesto/sample/geweke_test.py:202: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)
    alpha2[idxs[i]] = alpha2[idxs[i]] / (

Fixes #1207

[codecov-commenter]

Codecov Report

Attention: 417 lines in your changes are missing coverage. Please review.

Comparison is base (160c2a8) 88.16% compared to head (b71a171) 81.52%.
Report is 444 commits behind head on develop.

Files	Patch %	Lines
pypesto/ensemble/ensemble.py	68.46%	70 Missing ⚠️
pypesto/ensemble/util.py	55.04%	49 Missing ⚠️
...ypesto/hierarchical/spline_approximation/solver.py	83.68%	47 Missing ⚠️
pypesto/history/base.py	84.76%	32 Missing ⚠️
pypesto/hierarchical/optimal_scaling/solver.py	93.53%	26 Missing ⚠️
pypesto/hierarchical/petab.py	84.56%	23 Missing ⚠️
pypesto/engine/mpi_pool.py	0.00%	22 Missing ⚠️
pypesto/hierarchical/inner_calculator_collector.py	89.04%	16 Missing ⚠️
...pesto/hierarchical/spline_approximation/problem.py	89.18%	16 Missing ⚠️
pypesto/hierarchical/optimal_scaling/calculator.py	75.86%	14 Missing ⚠️
... and 19 more

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1217      +/-   ##
===========================================
- Coverage    88.16%   81.52%   -6.65%     
===========================================
  Files           79      148      +69     
  Lines         5257    11826    +6569     
===========================================
+ Hits          4635     9641    +5006     
- Misses         622     2185    +1563     

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