Perform a fit while stepping the value of a parameter through a
given range. Useful for determining confidence ranges in situations where
greater control is needed than given with the error command.
XSPEC> steppar <step spec.> [<step spec.>...]
where <step spec.> ::= [LOG or NOLOG] <param index> <low value>
<high value> <no. steps>.
The indicated parameter is stepped from <low value> to
<high value>
in <no. steps>+1 trials. The stepping is either linear or log. Initially,
the stepping is linear but it can be changed by the optional string
log before the parameter index. nolog will force the stepping to
be returned to the linear form. The number of steps is set initially to
10. At each value, the parameter is frozen, a fit performed, and the resulting
value of chi-squared given.
If multiple <step spec.> are given for different parameters, then a raster scan of the parameter ranges is performed. At the end of the set, the parameters and chi-squared are restored to the values they had initially.