Calculate 'timeInterval' instead of setting it as input

[limnoliver]

timeInterval is the minimum time expected between tips. This can be variable by rain gauge, and so may be more practical to calculate something like min(diff.time(pdate[i], pdate[i-1])) and use it as input to functions.

Currently implemented in RMevents_sko -- use elsewhere where timeInterval is needed.

[limnoliver]

In erosivity function, we need to internally calculate timeInterval, but we don't have the original (raw) data to calculate df(time). Export df(time) somehow from RMevents.

[limnoliver]

This is now an output of RMevents -- still deciding if this should still be an argument in the function where the user can subset the list from RMevents or just use behind the scenes.

[rbcarvin]

This can be behind the scenes, if the user changed it to something other than the minimum time then rainmaker will be giving them a less correct answer. In the past, the program 'trusted' the user to know this value, much better to just figure it out for the user.