Meta indicators for the IIASA Scenario Explorer

[EmiFej]

A recent change in the pyam package deactivated the ability to write a meta sheet when converting results to the IAMC template if no meta indicators are part of the workflow.

Meta indicators should be added so the results uploaded to the IIASA Scenario Explorer do not encounter errors due to missing meta sheets.

Should this addition be done in the OSeMBE_ECEMF repository or the osemosys2iamc (given that's where the conversion from .csv to IAMC format .xlsx takes place)? @willu47