v1.0.3 - index build code optimization

removed unnecessary operations and support multi-thread

• removed BWT building in bwa-meme index -a meme, which is unnecessary
• support for multi-threaded index build: .suffixarray_uint64, .pos_packed, .possa_packed, .ref2sa_packed
• removed requirement for .suffixarray_uint64 file at runtime
• usage info update in bwa-meme index