expfac: exponential modification

An exponential modification of a spectrum.

$$M(E) = \begin{array}{ll} 1+Aexp(-fE) & E > E_c\\ [.2cm] 1 & E < E_c \end{array}$$

where:

par1=$A$	amplitude of effect
par2=$f$	exponential factor
par3=$E_c$	start energy of modification