Adapt mat_vec_filler and filler_kernel for stencil format
Adapted the mat_vec_filler and filler_kernel files for use with matrices and vectors in stencil format:
- changed the base functions in filler_kernel to take ie1,ie2,ie3, and starts, pads as arguments
- changed the V0 and V3 function in mat_vec_filler analogously as the ones in filler_kernel
- changed the arguments when calling filler_kernel in mat_vec_filler
- updated documentation for both files
- fixed bugs in psydac_api/psydac_derham (now computes the number of cells correctly) and in example_psydac_parallel
- renamed tests for mat_vec_filler and filler_kernel to xx_test_ because they obviously will not pass anymore
This fixes clapp/hylife#48