Move sequential code generation to new host.Arg base class
Move OpenMP code generation to openmp.Arg, inherit host.Arg
No more need to specialize c_kernel_args for OpenMP
Remove specialization of OpenMP c_addto_{scalar,vector}_field
Remove specialization of OpenMP tmp_decl, c_zero_tmp
Use PETSc4py to assemble matrices in OpenMP
Remove assemble_mat from mat_utils
Break sequential code generation into more manageable chunks
Break OpenMP code generation into more manageable chunks
Move ParLoop.build to base class in host
Move code generation to common base class, OpenMP extends it