Allow chromosomes to be provided as individual FASTA files

[amandaclare]

I'd like to be able to supply individual files rather than one large FASTA file. For example:

sequence_data = { "mysample": {
        "chr1": {"file": "./chr1.fna.fai"},
        "chr2": {"file": "./chr2.fna.fai"},
        "chr3": {"file": "./chr3.fna.fai"},
        "chr4": {"file": "./chr4.fna.fai"},
        "chr5": {"file": "./chr5.fna.fai"},
}}
g = Goldilocks(GCRatioStrategy(), sequence_data, length="500K", stride="100K", is_faidx=True)
g.plot(group="mysample", title="GC Content")