Implementation of PMP ENSO metrics

[lee1043]

The following ENSO metrics will be integrated into REF through PMP:

Group	Name	Name2	Variable(s)	CMIP variable(s)
Teleconnections	DJF_PR_teleconnection	EnsoPrMapDjf	precipitation (PR), sea surface temperature (SST)	pr, ts
	JJA_PR_teleconnection	EnsoPrMapJja	precipitation (PR), sea surface temperature (SST)	pr, ts
	DJF_TS_teleconnection	EnsoSstMapDjf	surface temperature (TS)	ts
	JJA_TS_teleconnection	EnsoSstMapJja	surface temperature (TS)	ts
Physical processes	SST-Taux_feedback	EnsoFbSstTaux	sea surface temperature (SST), zonal wind stress (Taux)	ts, taux
	Taux-SSH_feedback	EnsoFbTauxSsh	sea surface height (SSH), zonal wind stress (Taux)	zos, taux
	SSH-SST_feedback	EnsoFbSshSst	sea surface height (SSH), sea surface temperature (SST)	zos, ts
	SST-NHF_feedback	EnsoFbSstThf	sea surface temperature (SST), net surface heat flux (NHF; sum of latent and sensible heat fluxes and longwave and shortwave radiations)	ts, hfls, hfss, rlds, rlus, rsds, rsus
	ocean_driven_SST	EnsodSstOce	sea surface temperature (SST), net surface heat flux (NHF; sum of latent and sensible heat fluxes and longwave and shortwave radiations)	ts, hfls, hfss, rlds, rlus, rsds, rsus

[lee1043]

Note: metrics using zos can be excluded for the first version as the variable not included for reference datasets.

[lewisjared]

Implemented in #273