Cache downloaded header file(s) in isigopen

[bemoody]

The usual idiom is to call isigopen(record, NULL, 0) to determine how many signals are in a record, then to call isigopen a second time after allocating the signal information buffer. In this case, we should only have to download the master header file (and the layout header file, if any) once. This could be implemented by keeping the WFDB_FILE handles open and rewinding them, rather than closing and reopening them.