Generate multisegment record

[bemoody]

When a record is finalized, we need to generate a multi-segment header file for it.

Up until now I've been doing this by hand using a hacked version of 'wfdbjoin', but this should ideally be done by the WaveSampleHandler itself.

This requires re-reading the segment header files, and creating a layout header with the composite signal information, and a master header containing the names and lengths of the segments (and gaps, if any.)