linked model list modtab. The time used here grows approx.
quadratically with the number of models. If the numbers get large,
e.g. with the IHP Open PDK, setup time make simulation impractical.
Therefore the linked list modtab has been enhanced by a hash table
modtabhash, which allows linear search time and makes simulation
of a 200k transistor circuit with IHP PDK possible. Already
the setup time for 15k transistor circuits has been reduced.