Add two more OpenMP versions

yee_naive_omp: simple (easy) implementation that performs poorly.

yee_stride1_omp: let the threads compute every other cell.
rename: py_field_variable
rename: py_field
rename: py_cell_partition
rename: py_alloc_field
rename: py_free_field
rename: py_set_grid
rename: py_vec_func
rename: py_vec_func2d
rename: py_apply_func
rename: py_init_acoustic_field
rename: py_init_local_acoustic_field
rename: py_acoustic_field
rename: py_assign_to
rename: py_get_from
rename: py_set_boundary
rename: py_leapfrog
fix rename mistake py_leapfrom
rename: py_timestep_leapfrog
rename: py_partition_grid