KLMixture: Use ctor with num of elements

Actually reserve was not quite correct, should have been resize. But we can as well use the vector ctor for nn elements

This merge request affects 1 package:

• Tracking/TrkFitter/TrkGaussianSumFilter

Adding @amorley as watcher

added Egamma Tracking master review-pending-level-1 labels

CI Result SUCCESS (hash 3d670fe2)

	Athena	AthSimulation	AthGeneration	AnalysisBase
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 18895]

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit d7f36dc3

added sweep:ignore label