user guide and exponential distribution

[ddalthorp]

"The user guide explains that the scale parameter in the exponential distribution will always be 1 by definition. Could this be reflected by the scale model field being removed under the Table and figure selection heading if an exponential distribution n is selected?"

Address this issue in the user guide explanation rather than the dropdowns.

[mhuso]

This is more elegant, but might cause minor problems if later someone is trying to put table together from other models.

[ddalthorp]

Scale for exponential in tables now reads NULL and User Guide has been updated too.