Incremental EigCG solver added

Hi Carsten,

This version has the Incremental EigCG (CG+deflation) for multiple right-hand sides added to the most recent version of tmLQCD in etmc/tmLQCD. The input parameters for EigCG is read in the input file and not used as global parameters. Here is a breif summary of the changes made:
1. The solver functions are in ./solver
2. Added a struct called solver_params_t in solver/solver_params.h. 
3. In operator.h the operator has now a member struct "solver_params". Currently, I use this struct to only store the eigCG solver parameters. I think we can move all the solver parameters there.
4. In read_input.l, I add lines inside the operator block to read the new solver parameters needed. 
5. There is a sample input file sample-input/sample-incr-eigcg-invert.input.

I have run this test and everything seems to work ok.

The meaning of the new solver parameters are explained in solver/solver_params.h

Cheers,
Abdou