Refs #4394 moved initialization steps from constructor to init
Refs #4394 Moved ElasticDiffSphere inside of DiffSphere
Refs #4394 Moved some initialization steps to init

These changes for the elastic part
Refs #4394 Moved inelastic part under DiffSphere files
Refs #4394 Some refactoring
Refs #4394 redoing the unit test
Refs #4394 Explicit check of boundaries
Refs #4394 correct silly format syntax
Refs #4394 Wrote four unit tests
Merge branch 'feature/4394_diffusion_sphere' into develop