Log Message:
Upgrade cad/ng-spice to v31.

Changes are probably many. From upstream's changelog:

2017-08-25  h_vogt
This ChangeLog is no longer maintained.
Starting with ngspice-26, for code changes
please refer to the git repository at
https://sourceforge.net/p/ngspice/ngspice/ci/master/tree/.
Especially the 'history' button on the top of the page
will give all the change log information.