+ cosmetic changes

OP_LinearOperator.hpp --> LinearEpetraOperator.hpp

markers_base.hpp --> Marker.hpp
markers_base.cpp --> Marker.cpp

Marker_Base --> Marker
MarkerTraits_Base --> MarkerTraits

DataMesh --> MeshData
dataMesh.*pp --> DataMesh.*pp

renamed files geo*->Element*

renamed files geo*->Element*

It would be better if the headers in each group are placed one
per line.