suggested fix for writing and reading aux arrays

I had a problem reading aux for plotting. I noticed some inconsistency between
writing and reading aux in ascii.py and petsc.py.

For ascii:
When I set the controller flag to write_aux_init=True, the file
fort.a0000 is being written. But when I read, the function read_ascii will
look for fort.a000(frame_id) or fort.a (I do not think that any code produce a file with name fort.a), so for frames other than the first
one, aux will not be read.

For petsc:
The aux arrays are always written in the file claw_aux.ptc and claw_aux.info. even
if I set write_aux_always to be true, the aux data are being overwritten.

I just tried to make the two codes acting the same (more like the ascii code), but the reading
function will look into fort.a000(frame_id) then into fort.a0000 in case the former is not found.