Avoid truncating ephemerides with repeated blocks

Before, an ephemeris that (a) had a repeated block of coefficients,
where (b) some of the coefficients did not match, would be silently
truncated at that block instead of getting completely compiled to numpy
arrays.  I have rebuilt DE421 since the earlier version of it that I
posted was accidentally truncated by this code.
Correct and streamline the DE421 package metadata