Automate iterative data format conversion

[GabrielKS]

When PowerSystems encounters old serialized data formats, it automatically converts them to the latest version. Roughly, the way this is currently implemented is with a function that converts the previous major version to the current major version and functions that iteratively compose other conversions from these (e.g., convert 1.0.0 to 3.0.0 is convert 1.0.0 to 2.0.0 followed by convert 2.0.0 to 3.0.0). Minor versions are handled ad hoc. This is manageable for now, but the number of conversion functions to be written for full coverage increases as the square of the number of formats. Most of these conversion functions will be the trivial composition kind. A better approach might be to replace those with some code that automatically finds the best "path" of conversions between one version and another. Extremely low priority, but it could be a good first issue for a new contributor to tackle.

[jd-lara]

It is kind of a stretch to call this a good first issue.