Overwriting datasets

[stuartmcalpine]

When datasets can be overwritten, does this work with directories?

I don't know if i've ever tried registering a directory that can then be overwritten, will it properly overwrite the directory, or will some files be left over if the exact structure isn't the same.

Need to check how copytree(old_location, dest, copy_function=copyfile) works, maybe we have to delete the previous folder first.

[stuartmcalpine]

Address this in #86