#73 Versioning of XSPICE code models

[Jim Monte]

To fix memory issues in some code models, it was necessary to get more functions from ngspice. That introduced a problem of how a version of ngspice and a code model would know what each other supports. This issue was addressed with versioning support. An approach is provided as part of commit da9bcc050f286813994b59279db65d5ab74c2d78 in branch beta_jdm. More details are provided in the attached files. The two attached files are the same except for file format.