Log Message:
erfa: update to 2.0.0.

Based on update in wip by Kinoshita Daisuke, nia and mef.

This version of ERFA (v2.0.0) is based on SOFA version "20210512",
with the differences outlined below.

ERFA 2.0.0 and SOFA "20210512"

    Functionally, there no differences except for the added
    eraVersion, eraSofaVersion, and leap second functions noted
    above.
    The three new functions added in SOFA are added to ERFA as
    well.
    Like for SOFA, one now has to explicitly include erfam.h to
    use its macros defining constants, etc. Including just erfa.h
    (or erfaextra.h) will no longer includes erfam.h implicitly.