Preliminary implementation of PETSc using GPUs

PETSc is a scalable solver library for the solution of algebraic equations arising from the discretization of partial differential equations and related problems. PETSc is organized as a class library with classes for vectors, matrices, Krylov methods, preconditioners, nonlinear solvers, and differential equation integrators. A new subclass of the vector class has been introduced that performs its operations on NVIDIA GPU processors. In addition, a new sparse matrix subclass that performs matrix-vector products on the GPU was introduced. The Krylov methods, nonlinear solvers, and integrators in PETSc run unchanged in parallel using these new subclasses. These can be used transparently from existing PETSc application codes in C, C++, Fortran, or Python. The implementation is done with the Thrust and Cusp C++ packages from NVIDIA.