[CMake] eigenpy is required at runtime

If libeigenpy.so is available at buildtime, and not at runtime, the
python bindings can't work.
20 jobs for master in 10 minutes and 46 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
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passed format #61977

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passed robotpkg-multicontact-api-16.04-debug #61959

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passed robotpkg-multicontact-api-16.04-release #61958

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passed robotpkg-multicontact-api-18.04-debug #61961

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passed robotpkg-multicontact-api-stretch-debug #61963

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passed robotpkg-multicontact-api-stretch-release #61962

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passed robotpkg-py-multicontact-api-16.04-debug #61965

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passed robotpkg-py-multicontact-api-16.04-release #61964

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passed robotpkg-py-multicontact-api-18.04-debug #61969

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passed robotpkg-py-multicontact-api-18.04-release #61968

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passed robotpkg-py-multicontact-api-py3-16.04-debug #61967

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passed robotpkg-py-multicontact-api-py3-16.04-release #61966

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passed robotpkg-py-multicontact-api-py3-18.04-debug #61971

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passed robotpkg-py-multicontact-api-py3-18.04-release #61970

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passed robotpkg-py-multicontact-api-py3-stretch-debug #61975

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passed robotpkg-py-multicontact-api-py3-stretch-release #61974

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passed robotpkg-py-multicontact-api-stretch-debug #61973

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passed robotpkg-py-multicontact-api-stretch-release #61972

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