Variables for production of cement and steel

[danielhuppmann]

The @IAMconsortium/common-definitions-sdg-indicators group in #53 suggested to add the following variables that are directly related to the domain of the @IAMconsortium/common-definitions-macro-economy group.

- Production|Cement:
    definition: production of cement
    sdg: 9
    unit: Mt/year
- Production|Steel:
    definition: production of steel
    sdg: 9
    unit: Mt/year
- Production|Steel|Primary:
    definition: production of primary steel
    sdg: 9
    unit: Mt/year
- Production|Steel|Secondary:
    definition: production of secondary steel
    sdg: 9
    unit: Mt/year
- Value Added|Industry|Other Industry:
    definition:
    sdg: 9
    unit: billion US$2010

[orichters]

Sounds fine from my side. But please use /yr, not /year, also here.

[orichters]

In NGFS, we also use:

- Production|Chemicals:
    definition: production of chemicals
    unit: Mt/yr
- Production|Chemicals|Other:
    unit: Mt/yr
- Production|Non-ferrous metals:
    definition: production of non-ferrous metals (e.g., aluminum)
    unit: Mt/yr
- Production|Plastics:
    unit: Mt/yr
- Production|Pulp and Paper:
    unit: Mt/yr