Reading MS signal metadata in rdheader

[bemoody]

In general, it's useful to be able to read record metadata without reading the signals.

For multi-segment records, however, rdheader just reads the MS header by default (rd_segments=False), which means it doesn't read any of the signal metadata (e.g. signal names and units).

With rd_segments=True, rdheader reads the signal metadata and also all of the segment headers, which are not always needed.

For some applications it would be useful to have a middle ground of reading the multisegment header and the layout header, but not any of the segment headers. (Note that this is what isigopen does.)

I think this might make sense as the default behavior, since reading the signal metadata is a very common use of rdheader.