IGS ERP UT1-TAI support

[ourairquality]

A suggestion to also support UT1-TAI in the IGS ERP format. It's probably simpler to just support this than to detect and then ignore this data if not UT1-UTC - once detected it appears trivial to apply an offset.

The IGS ERP format appears to allow the UT1-UTC, UT1R-UTC, UT1-TAI, and UT1R-TAI and it does appear that UT1-TAI is commonly produced, see the example below from GFZ.

version 2
GFZ    ERP-RESULTS for WEEK 2334       DAYS  272 - 279
_______________________________________________________________________________
  MJD       Xpole   Ypole   UT1 -TAI   LOD  Xsig Ysig UTsig LODsig Nr Nf Nt  X-RT  Y-RT X-RTsig Y-RTsig
             E-6"    E-6"    .1 us  .1 us/d E-6" E-6" .1 us .1 us/d          E-6"  E-6"    E-6"    E-6"
60582.500  225415  411006 -369409348   -301    4    4   11   11   128 99 78   520 -2158      16      16

Here is a product from CODE using UT1-UTC for the same date:

VERSION 2
CODE MGEX 3-DAY SOLUTION, DAY 275, YEAR 2024                     04-OCT-24 22:21
-------------------------------------------------------------------------------------------------------------------------------------------------
NUTATION MODEL       : IAU2000R06               SUBDAILY POLE MODEL: DESAI2016
  MJD         X-P      Y-P   UT1UTC    LOD   S-X   S-Y  S-UT  S-LD  NR  NF  NT   X-RT   Y-RT  S-XR  S-YR C-XY C-XT C-YT   DPSI   DEPS  S-DP  S-DE
              E-6"     E-6"    E-7S E-7S/D   E-6"  E-6" E-7S E-7S/D                 E-6"/D      E-6"/D    E-2  E-2  E-2    E-6"   E-6"  E-6"  E-6"
60582.50   225338   410967   590619    245     6     6     2     7 147  85 121    275  -2164     9     8    3    1   -5      0      0     0     0

The difference is 37 seconds, the difference between UTC and TAI at present, so the logic appears simple. Might just search for TAI in a header line and flag this offset to be applied if found.