make emissivity a namelist variable

[jonbob]

It would be very helpful for E3SM if the ice emissivity could be set via a namelist instead of being a parameter. The correct value of this seems to be a research subject and it would help to be able to test different values a bit more easily.

[apcraig]

emissivity is a setable parameter in icepack. it is set by this call,

call icepack_init_parameters(emissivity_in=value)

icepack itself has no namelist. E3SM should have it's own driver, namelist, and ability to set this parameter in icepack.

We could add emissivity to the icepack driver namelist and/or to the CICE namelist, but that probably would not have an impact on E3SM. @eclare108213, please let me know if that would be helpful and then which namelist we should use.

[eclare108213]

I agree, E3SM can set this up as a namelist parameter regardless of what we do with it.
It would be nice to offer it as a namelist parameter for both Icepack and CICE drivers. I'd probably put it in forcing_nml. Open to other suggestions for that.

[apcraig]

see #213 for icepack. same coming soon for CICE.

[apcraig]

CICE PR 176.