GCSA2 v0.4

This is the fourth pre-release of GCSA2. The memory usage of index construction is now significantly lower, at the price of 2x longer construction time.

• The construction algorithm is now disk-based. It keeps the graphs on disk and loads at most one chromosome at a time into memory.
• Graph order is no longer a hard limit for query length. Longer queries may still result in false positives, however.
• Headers have been reorganized into public headers (gcsa.h, files.h, support.h, and utils.h) and internal headers (path_graph.h, dbg.h, internal.h).