Add a task parameter to control the time period to which the
GTI data set is applicable, e.g. a single exposure instead of the entire
observation. This will speed up the task's execution time but has no other
substantial benefits.
Support a more powerful syntax for the expressions in the
overrideparametervalues parameter. E.g. @1, @2,
etc. refer to the names of the first, second, ..., HK parameter given
in overrideparameters.
