Explicit user-exposed Series close/(re-)open functionality

Currently, the lifetime of file handles used in the backends is tied to the lifetime of a Series they are contained in. It would be very convenient to offer explicit control over this lifetime by having a public interface to close all handles in a Series (e.g. through Series::close() and likewise to re-open a closed Series (e.g. through Series::reopen()). Sidenote: When reopening a file series, it would be wise to re-read the entire file the meta-data of all involved files instead of resuing old values stored in this logical API. The reasoning is that the file could be modified externally, resulting in an incosistent state between logical and physical representation.