Write scientific defaults upon close (Iteration::close or Series::close)

[franzpoeschel]

Close #1439
This PR focuses on Iteration-level attributes (including attributes included below Iteration level)
Needed for introducing a synchronous mode

TODO:

• Write side changes
• Read side changes: Move attribute initializations to new ScientificDefaults logic as well, be more lenient about missing attributes
• Maybe expose the finalize() method publically, so users can request openPMD to set defaults now
• Reduce use of templates, size of ScientificDefaults.o
• Check places from where readDefaults is called
• particlePatches being weird, check them
• clean up declarations and definitions
• Decide if defaults should be populated upon flush() (benefit: early readers get readable datasets) or upon close() (benefit: we do populate too eagerly, users might add defaults late)
• Do sth about populateDefaultMetadata()

[franzpoeschel]

TODO: Move this check into finalize, maybe only print a warning

            throw std::runtime_error(
                "A Record can not be written without any contained "
                "RecordComponents: " +
                name);

[ax3l]

@franzpoeschel is this for 0.18.0? :)

[franzpoeschel]

@franzpoeschel is this for 0.18.0? :)

Yup, this is still under development

[ax3l]

@franzpoeschel sorry small conflict in CoreTest.cpp, will review the rest in the meantime :)

[franzpoeschel]

TODO: verify that this properly implements the previous logic for require_unit_SI