Adapt the mapping for DAT_SURFACE_TEMP towards 'Surface Air Temperature Change'

[maltemeinshausen]

https://github.com/openscm/pymagicc/blob/b6e9c1a34b85ff45b92be8fd03e43d601eaba06e/pymagicc/definitions/init.py#L463

The current mapping to 'Surface Temperature" does not seem to mach openscmrunner conventions.

[znicholls]

@lewisjared now we have users finding problems with this too

[lewisjared]

Welcome @maltemeinshausen 😄

I'm guessing the approach would be to support both Surface Temperature and Surface Air Temperature Change temporarily, because not handling Surface Temperature would break things...