Castro 19.09

-- You can now type ./Castro.gnu.ex --describe to see the list of
modules / compilers the code was built with (#660)

-- The reaction quantities are now computed as proper 4th order
averages for the plotfile, when using sdc_order = 4 (#647)

-- The velerr tagging now takes the abs() of the velocity component
to ensure we tag large positive and negative velocities.