Fix pkg/shelfice saltFreeze=0-bug (#968)

• avoid division by saltFreeze = 0

• can happen when sLoc = 0 (reported by D. Menemenlis)

• add alternative computation of fw-flux copied from stic_solve4fluxes

use this code by defining SHELFICE_USE_HEATBALANCE_FOR_FRESHWATERFLUX

• simplify if statements and make AD-code use new formulation

also fixes new recomputation warning for TAF

• update documenation and attempt to “improve” description of 3 eqn.

• fix a bug for advDiffHeatFlux=T and adjust/expand the documentation

• seems like there is no effect on the solution (surprisingly)

• small adjustments to text

• more tweaks and a typo

• fix doc typos

• improve documentation following Ou’s comment, fix cpp-flag

• replace long flag by shorter (undef) SHI_SALTBAL_FWFLX; this also changes the default, so that now we use the heat balance to diagnose the freshwater flux (more robust)

• tweak documentation to describe SHI_SALTBAL_FWFLX properly

• fix bug: incldue term “- a0*(eps1+eps3)*SHELFICEsalinity”

• found by Ou Wang
• more small changes based on Ou’s review

• replace upper case I,J with lower case i,j

• update all SHELFICE_OPTIONS.h, use new flag SHI_SALTBAL_FWFLX

in shelfice_2d_remesh; small change to upper/lower case in shelfice_thermodynamics.F

• replace few “K” with “k”

there was many more “k” than “K”, write it in a consistent way

• document pkg/shelfice fix