Consider being more flexible with new format ID fields

[ross-spencer]

The changes to #19 persist a certain amount of rigidity with SF fields. In another implementation, we can simply ignore changes to the Siegfried structure - other's might say knowing about changes is useful, but we shouldn't have to react to changes that don't effect the overall functionality of the code.

We can:

1. Ignore issues with ID imports by default.
2. Have a strict mode that measures conformance with a schema.

Really, ID tools change so little, and this code is used only by myself, so it's not a huge priority.