* Adapt docn_datamode_cplhist for dice_datamode_cplhist

* Fix .or. and local  variables have different names in docn and dice

* Add Si_imask. Decide whether to have flds_i2o_per_cat for cplhist method.

* Copy sea ice stream fields to export state fields via dshr_dfield_add

* Switches So_t from C to K. Decide on which/add switch

* Overwrite missing value of 0 K in Si_t

* Handle So_t in C and K in docn_datamode_cplhist_mod.F90

Need model SST to be in K
if (minval(So_t) .LT. 100.0_r8), convert C-->K

* Only fill docn_datamode_cplhist export variables if associated. Handle units of So_t based on namelist input

* Default convert SST C to K (as currently). Then switch based on optional sst_constant_value

* Allow null return for taux too in dice/dice_datamode_cplhist_mod.F90

* Add doc for dice cplhist mode to dice.rst

…c.F90

errors are handled internally in that subroutine and do not return