gplepage/corrfitter: corrfitter version 6.0

Substantial rewrite of most of the package, though the functionality is mostly the same. The new code is also mostly backwards compatible, but has some new features, and is more robust and better tested than before. The new features include: coarse-graining of data, to reduce its size; simpler specification of tmin, etc. in correlators; new keywords reverse and reverseddata in Corr2 and Corr3 that facilitate handling time-reversed data (e.g., for combining a->V->b with b->V->a); support for reading datasets from hdf5-formatted files; and new options for chained fits. This version requires lsqfit v9.1.1 and gvar v8.2 or later.