DataContainer reads wrong offset

[ax3l]

When reading a domain with DomainCollectors putting the result via addDomain to a DataContainer the offset in the container is determined falsely (always 0,0,0).

logic error in libSplash when reading offsets:

• default = 0,0,0 https://github.com/ComputationalRadiationPhysics/libSplash/blob/563da86e62fc483d214bed4163140b15f4957abe/src/include/splash/domains/DataContainer.hpp#L48
• min with values > 0 will never be the correct offset: https://github.com/ComputationalRadiationPhysics/libSplash/blob/563da86e62fc483d214bed4163140b15f4957abe/src/include/splash/domains/DataContainer.hpp#L86

Related to https://github.com/ComputationalRadiationPhysics/picongpu/issues/1544

[Flamefire]

Idea for a fix: If the first entry is added then init offset to the entrys offset. Would only be 2 lines added.

[ax3l]

that could work, but I did not check the full logics it will touch yet.

maybe we add a (currently failing) test first and then implement that by checking subdomains.size() for the first add?