Invert Linear Model
CALL sla_INVF (FWDS,BKWDS,J)
FWDS  D(6)  model coefficients 
BKWDS  D(6)  inverse model 


J  I  status: 0 = OK, $$1 = no inverse 
${x}_{2}=a+b{x}_{1}+c{y}_{1}$
${y}_{2}=d+e{x}_{1}+f{y}_{1}$
${x}_{1}=p+q{x}_{2}+r{y}_{2}$
${y}_{1}=s+t{x}_{2}+u{y}_{2}$