Support intermediate expr variables

[goord]

Many soil-related expressions are much more readable if we would support intermediate variables, e.g. mrsow

(var39 * 0.07 + var40 * 0.21 + var41 * 0.72 + var42 * 1.89 - th_wilt) / (th_sat - th_wilt)

where

th_wilt = (var43==1)*0.059+(var43==2)*0.151+(var43==3)*0.133+(var43==4)*0.279+(var43==5)*0.335+(var43==6)*0.267+(var43==7)*0.171)

and

th_sat = ((var43==1)*0.403+(var43==2)*0.439+(var43==3)*0.430+(var43==4)*0.520+(var43==5)*0.614+(var43==6)*0.766+(var43==7)*0.472)

Given the length of the above expressions, it would be really convenient to be able to write in the ifspar.json

"expr" : "(var39 * 0.07 + var40 * 0.21 + var41 * 0.72 + var42 * 1.89 - th_wilt) / (th_sat - th_wilt); th_wilt = ... ; th_sat = ..."

which should by ece2cmor3 be translated to the cdo command

cdo -expr,"var46 = (var39 * 0.07 + var40 * 0.21 + var41 * 0.72 + var42 * 1.89 - th_wilt) / (th_sat - th_wilt)" -aexpr,"th_wilt=..." -aexpr,"th_sat=..."

[klauswyser]

Very good suggestion! Hope you can make it work.