Clean up API for ChElementBrick.
Clean up API for ChElementBrick.

- hide implementation from public API
- make data private (not protected)

tjolsen/chrono,amelmquist/chrono,rserban/chrono,andrewseidl/chrono,rserban/chrono,tjolsen/chrono,projectchrono/chrono,Bryan-Peterson/chrono,projectchrono/chrono,armanpazouki/chrono,Bryan-Peterson/chrono,Milad-Rakhsha/chrono,dariomangoni/chrono,armanpazouki/chrono,armanpazouki/chrono,Milad-Rakhsha/chrono,dariomangoni/chrono,dariomangoni/chrono,projectchrono/chrono,armanpazouki/chrono,rserban/chrono,Bryan-Peterson/chrono,Bryan-Peterson/chrono,jcmadsen/chrono,Milad-Rakhsha/chrono,projectchrono/chrono,tjolsen/chrono,armanpazouki/chrono,andrewseidl/chrono,jcmadsen/chrono,amelmquist/chrono,rserban/chrono,Milad-Rakhsha/chrono,dariomangoni/chrono,jcmadsen/chrono,amelmquist/chrono,jcmadsen/chrono,amelmquist/chrono,tjolsen/chrono,jcmadsen/chrono,andrewseidl/chrono,dariomangoni/chrono,andrewseidl/chrono,jcmadsen/chrono,Milad-Rakhsha/chrono,projectchrono/chrono,amelmquist/chrono,dariomangoni/chrono,jcmadsen/chrono,amelmquist/chrono,andrewseidl/chrono,Milad-Rakhsha/chrono,rserban/chrono,rserban/chrono,tjolsen/chrono,Bryan-Peterson/chrono,rserban/chrono,armanpazouki/chrono,projectchrono/chrono