Add parameters so user can enter some measurable quanitites

[jessecarterMOOSE]

For SinkUserObject:
Allow user to specify "dislocation density", and imply spacing and strength from that, using basic rate theory principles.

For EventInserter:
Allow user to specify "flux" and "normalized dpa rate" and others to compute the mean cascade frequency.

This will likely involve splitting up these classes into Base and Derived classes