Index / load sequences from FASTG files

[fedarko]

Various ideas:

• Very lazy option: convert FASTG to FASTA, which can then be loaded / indexed with various tools
• The FASTG file can probably be indexed "as is" using various FASTA indexing tools -- will just need to split sequence IDs from a:b,c or something to get just a
• Do the parsing ourselves entirely. While we go through the file, just store the start / end coordinates of each sequence within the file -- then we can programmatically access these sequences on demand later