"extresist" command continues to work as before. However, the
method now reads from .ext files instead of .sim files, so
generating ".sim" and ".nodes" files is no longer necessary.
In addition, the core code of "extresist" was put directly into
ExtCell.c so that full R-C extraction can be run using
"extract do resistance" followed by "extract all", without
needing to run "extresist" at all other than to set parameters
(e.g., "extresist tolerance 10").