store hydrogen bonds between beta-strands

[jamesmkrieger]

The second part of the SHEET lines stores hydrogen bonds to previous beta-strands that indicate strand packing register e.g. 2tbv has the following.

SHEET    1 S1A 4 VAL A 103  ASN A 117  0                                        
SHEET    2 S1A 4 ASP A 250  TYR A 264 -1  O  LEU A 263   N  VAL A 103  

Strand 1 in sheet S1A has no second part. Strand 2 has a hydrogen bond to strand 1.

See http://www.wwpdb.org/documentation/file-format-content/format33/sect5.html#SHEET

There should probably be a common data structure that can also be used for interactions in #1292

[jamesmkrieger]

It should probably resemble the Bond class