v1.2.1

This release mainly introduces enhancement to the command-line arguments of pppx. Users can run pppx -v and pppx -h to get the software version and help information, respectively. Additionally, pppx -x <sol_mode> outputs defeault settings to stdout for the specified solution mode (e.g., spp/ppp/rtk/tdp), allowing users to redirect the output to generate a configuration file. It could be useful if a configuration file is not prepared.

Bug fixes:

1. Disable the error when spp fails but approximate station position is available in the RINEX file
2. Fix typos in the manual

Performance improvements:

1. Enhance command-line arguments: -v/--version, -h/--help, -x <sol_mode>
2. Support FGO-based RTK with ambiguity resolution
3. Output fixed information for PPP-AR in the .stat file when solver_type = ekf