Read/write index

[bkille]

• Implements index saving/loading
• --mm-index FILE will read the index saved at FILE if FILE exists, otherwise it will construct the index and write it to FILE for future use.
• The index can be overwritten with --overwrite-mm-index.
• wfmash will exit if the CLI parameters do not match those in the the saved index. In particular, if the segment length, sketch size, or kmer size are different.