Fault-tolerant handling of intermediate results

[aseeland]

Intermediate results assume an incremental number at the end of each file.
However, it can happen, that files are missing inbetween, due to missing (overwritten) stdout-lines, e.g. see benchmark case 4, tpfa-circ. Thus, the frontend should be able to handle missing files and include them in the graphics at the end of the computation.