[Doc] added linear solver docu

Documentation/ProjectFile/linear_solver/c_linear_solver.md

+In this group the configuration of linear solvers is described.
+
+Unlike all other options, there can be configurations for linear solvers from
+the Eigen, LIS, PETSc, etc. libraries at the same time. Which solver will be
+applied in the end is determined by the build configuration of the OGS
+executable.

Documentation/ProjectFile/linear_solver/eigen/i_eigen.md

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+Configuration for solvers from the Eigen library.
+
+For details see also the [documentation of the Eigen library](https://eigen.tuxfamily.org/dox/group__Sparse__Reference.html).

Documentation/ProjectFile/linear_solver/eigen/t_error_tolerance.md

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+The tolerance of the linear solver.
+
+This setting is only applied if an iterative solver is chosen.
+
+Its default values is \f$10^{-16}\f$.

Documentation/ProjectFile/linear_solver/eigen/t_max_iteration_step.md

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+Defines the maximum number of iterations of the iterative solver.
+
+This setting is ignored if a direct solver is chosen.
+
+Its default value is \f$10^6\f$.

Documentation/ProjectFile/linear_solver/eigen/t_precon_type.md

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+Chooses a specific preconditioner for the iterative linear solver.
+
+This setting is ignored if a direct solver is selected.
+
+Possible values are NONE, DIAGONAL and ILUT.
+
+The default is NONE.

Documentation/ProjectFile/linear_solver/eigen/t_solver_type.md

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+Chooses a specific linear solver.
+
+Possible values are CG, BiCGSTAB, and SparseLU.
+
+The default is SparseLU.

Documentation/ProjectFile/linear_solver/petsc/i_petsc.md

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+Configures options for linear solvers from the PETSc library.
+
+For further details see also the
+[documentation of the PETSc library](http://www.mcs.anl.gov/petsc/documentation/)

Documentation/ProjectFile/linear_solver/petsc/t_parameters.md

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+Sets PETSc options from the given parameter string.
+
+The default value is <tt>"-ksp_type cg -pc_type bjacobi -ksp_rtol 1e-16 -ksp_max_it 10000"</tt>
+
+\attention
+PETSc can also receive options from the commandline arguments. However, the
+values from the prj file are applied directly to the linear solver. If
+specifying both, make sure that you check which options take precedence.

Documentation/ProjectFile/linear_solver/petsc/t_prefix.md

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+Sets the options prefix for this PETSc linear solver.
+
+Cf. the PETSc [KSPSetOptionsPrefix documentation](http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetOptionsPrefix.html).

Documentation/ProjectFile/linear_solver/t_lis.md

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+Defines an option string that will be passed to solvers of the LIS library.
+
+The default is <tt>-initx_zeros 0</tt>, i.e., not to overwrite the passed initial
+guess with zeroes. All other options are appended to this default value.
+
+\attention
+LIS can also receive options from the commandline arguments. However, the values
+from the prj file are applied directly to the linear solver. If specifying both,
+make sure that you check which options take precedence.
+
+Refer to the [documentation of LIS](http://www.ssisc.org/lis/index.en.html) for further details.

Documentation/ProjectFile/linear_solver/t_name.md

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+Name of the linear solver.
+
+Used, e.g., to reference it in the nonlinear solver configuation.