New `sncl_mode` option for FDSNFactory (for EdgeFactory compatibility)

EdgeFactory (and MiniSeedFactory) recently added the sncl_mode option (possibly still in a MR). This should be added to FDSNFactory.

NOTES:

• FDSNSNCL.py should be git-renamed to something more like ASLSNCL.py to reflect that it applies ASL's standards;
• the accepted sncl_modes should be: {"legacy", "geomag", "asl"};
• assuming the updated EdgeFactory and MiniSeedFactory have been merged, add "asl" to the allowed sncl_modes for those factories;

Mentioning @swilbur since he's worked with this before and may have comments. I'm not necessarily assigning him.