add -sys flag for constellation selection

[CodeAndCoords]

Summary

RINEX files with corrupted observations on one constellation cause
SPP to return "no solution are computed" for the entire session,
even when the remaining constellations are healthy and sufficient
for positioning. There is currently no command-line option to
exclude a single constellation.

How to reproduce

1. Take a RINEX file where one constellation has corrupted
   pseudoranges (e.g. negative C1 on GLONASS).
2. Run SPP with the default constellation set (GREC):
   spp -trop saas -ts 2026/05/25 19:36:55.00000 -te 2026/05/25 22:55:13.00000 -ti 5.00 rover.obs brdc.rnx -o out.pos
   → returns "Info : no solution are computed, postpos()"
3. Run excluding the problematic constellation:
   spp -trop saas -ts 2026/05/25 19:36:55.00000 -te 2026/05/25 22:55:13.00000 -ti 5.00 -sys GEC rover.obs brdc.rnx -o out.pos
   → produces a valid solution over the full session.

Changes

• get_spp_args.f90 — -sys flag + IsSysToken helper
• dispinfo.f90 — updated -h

Usage

spp -trop saas -ts YYYY/MM/DD hh:mm:ss.00000 -te YYYY/MM/DD hh:mm:ss.00000 -ti 5.00 -sys GEC obs.rnx nav.rnx -o out.pos
G=GPS R=GLO E=GAL C=BDS J=QZS I=IRN S=SBAS
Default (no -sys): GREC

[CodeAndCoords]

Thank you for your continued work on PRIDE PPP-AR.
Happy to adjust anything if needed.
Best regards,
Juan