added handling of RNA alphabets in bits_per_symbols()
added handling of RNA alphabets in bits_per_symbols()

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518119]

kimrutherford/nvbio,NVlabs/nvbio,kimrutherford/nvbio,NVlabs/nvbio,NVlabs/nvbio,kimrutherford/nvbio,kimrutherford/nvbio,NVlabs/nvbio