+ cosmetic changes

OP_LinearOperator.hpp --> LinearEpetraOperator.hpp

reordering, documentation, readability

OP_*, nsipterms.hpp, sdStabilization.hpp,  by U. Villa
mesh*, basisElSH*, postProc.hpp by T. Passerini
bareItems*, identifier*, markers* by L. Bertagna
geo* by M. D'Elia

This is the result of the command

'astyle --style=allman -M80 -O -r *.*pp'

in the root folder of lifev.

The new namespace LifeV::Operator has been introduced.
All classes in this namespace make intesinve use of Trilinos raw data structure.

Up to now a virtual abstract class LinearOperator has been written to provide
a common interface to all the derived classes. LinearOperator inherit from
Epetra_Operator.

Basic derived class from LinearOperator are IdentityOperator and NullOperator.

A less trivial derived class is BlockOperator. This class allows to define
block structured matrices and preconditioners.