GpuRecoSts: Small fixes.

This MR provides two small fixes in the GPU STS Reco:

• Ensure xpu::initialize() is only called once during init
• Reset input digis between timeslices.

Dear @v.friese,

you have been identified as code owner of at least one file which was changed with this merge request.

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added CodeOwners label

added 5 commits

• 5cca6073...9117f2bf - 4 commits from branch computing:master
• 6654966d - GpuRecoSts: Small fixes.

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merged