add switches to override check_old_data_is_okay_to_use?

Both the grid and MCMC search classes have check_old_data_is_okay_to_use methods, which are a good idea in principle, but might need to get quite a bit more involved to check all possible differences between search calls in all use cases. (E.g. the simple grid output format currently cannot be used to distinguish between a search with and without t0,tau bands, which I'll open another issue about.)

So a nice convenience feature, at least for use during development, would be a 'reusedata' argument, default True, but when set to False just skips that check. If you like it, I can make a patch.