A proposal for cache validation

[Vinye]

I keep repeating the mistake of using old invalid cache without noticing it, so here is a simple proposal for validation method. Related to issue #10

[matiaslindgren]

Thanks! This looks good and will be efficient even for large dataframes. I was initially thinking about automatically deleting invalid caches, but making this an error is actually better since the user might not want to always delete old caches.