Merge branch 'matrix_product_operator' into 'NIFTy_5'

add operator to multiply fields with matrices

See merge request !323
11 jobs for NIFTy_5 in 19 minutes and 7 seconds (queued for 2 seconds)
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  Build Docker
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build_docker_from_scratch

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  Test
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test_mpi

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test_serial

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  Release
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pages

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  Demo Runs
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run_bernoulli

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run_curve_fitting

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run_getting_started_1

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run_getting_started_3

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run_ipynb

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  Deploy
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