checkpoint patches

[dcdpr]

Checkpoints should be immutable. But people will find old coins that did not invalidate prior tax returns but do affect the current return. So we need checkpoint patches.

Allowing this more-structured reconciliation also helps get rid of the hacky --input-basis option.

[dcdpr]

GL issue 157

[dcdpr]

Currently, we do not have any special support for migration from legacy checkpoint versions. This means that the most reasonable way to generate a checkpoint - for a prior year - in some new format (when some future release creates that new format) is to rerun all the prior years that have been assigned to taxcount. This will break the "checkpoints should be immutable" goal. However, it is a broader case than merely finding coins that were not bootstrapped properly (the initial scope of this issue).

Does that mean that this issue is hopeless, or that we should put equal effort into migration patches? I don't think it's easy to solve migration in the same way this issue contemplated for forgotten coins. I think that this does affect this issue's priority.