Added UserObject for J2 plasticity

This has strength = 1 hard-coded.

Three tests added.

Fixes #3952
Added J2 plasticity UserObject with Gaussian hardening

Two tests

Fixes #3952
Removing trailing spaces and tabs.  Fixes #3952
Merge pull request #3955 from WilkAndy/j2_uo_3952

J2 plasticity as a userobject