pdp3: warn on the console when the receiver antenna is missing from the ANTEX file

[smyrgl]

Problem: When the antenna type in the RINEX header has no entry in the ANTEX file, rdatx prints ###WARNING(rdatx): Antenna not found and applies a zero antenna model (no PCO, no PCV). But pdp3 runs the programs through ExecuteWithoutOutput, which discards their output when they exit 0, so that warning never reaches the user and the run looks clean.

I hit this with a SparkFun SFESPK6618H, which is calibrated by NGS but absent from igs20.atx. The processing succeeded and the height came out about 102 mm high compared with the same day processed after adding the NGS entry to the table copy of the ANTEX (horizontal position unaffected). Nothing in the console pointed at the antenna.

Fix: Right after Prepare IGS ANTEX file ... done, pdp3 reads the antenna type from the RINEX header (columns 21-40 of the ANT # / TYPE line) and looks it up in abs_igs.atx by an exact 20-column match on the TYPE / SERIAL NO lines. If it is missing it tries the radome NONE variant, mirroring what rdatx does itself, and says so. If that is missing too it prints two warning lines and a hint naming the table file to augment:

warning: PrepareProducts: antenna `SFESPK6618H     NONE' NOT FOUND in ANTEX file abs_igs.atx
warning: PrepareProducts: a ZERO antenna model (no PCO/PCV) will be applied, which biases the height by centimetres
:: please check the antenna type in the RINEX header, or add its calibration (e.g. from NGS) to /path/to/table/igs20_2425.atx

It is a warning only; processing is unchanged. The check is skipped in LEO mode, where leoatx.py adds the entry.

Verified: The same daily RINEX file processed twice on Ubuntu 24.04: with the stock igs20_2425.atx the warning is printed and the SITE ANTENNA PCO lines in the pos file are zero; with the NGS entry appended to the table copy there is no warning and the PCO values are applied. Antennas present in the stock table (for example TRM59800.00 SCIS) pass silently.