Update the diagnostics of global conservation

[cetlod]

The diahsb module currently perform global conservation of T/S only in the non linear free surface case where conservation is expected to be satisfied ( except if virtual salt flux is use between ice and ocean : nn_ice_embd = 0 ).
We add the linear free surface case. In this case the heat and salt conservation is not satisfied ( see Roullet and Madec , JGR 2000 ; Leclair & Madec 2009 ).
Typically, in non linear case, the conservation is about 10-8 for heat and salt content in for ORCA2-LIM2 ( nn_ice_embd = 1 o r2 ) and it can be even reduced if explicit vertical diffusion is used ( see Leclair & Madec 2009 )
In linear free surface the conservation can't be satisfied. The error is of the order of 10-3 (see Leclair & Madec 2009 ) if the heat and salt content included in the ssh is taken into account ; otherwise it's about 10-2