Add a DNA/RNA builder.

Uses Open Babel's FASTA format support.
In OB 2.3.1, this enables single/double strand
DNA and modifying the # of bp per turn.

Change-Id: I82e13b1ac9bd66eea18adadbd94dc7fc9c0793ff
Merge topic 'ENH_dna-builder' into master

1f9ad521 Add a DNA/RNA builder.