Exponential fit, without start-guess by Per Sundqvist

Fits 1) f=s1+s2*exp(-t/s3) or 2) f=s1+s2*exp(-t/s3)+s4*exp(-t/s5) to numerics, without startguess (exponential, nonlinear, initial guess)

s=exp2fit(t,f,caseval,lsq_val,options)