Deprecate some arguments in `start_scan` and refactor fallback seed

[mlincett]

The following arguments of start_scan:

1. input_time_name: str
2. input_pos_name: str
3. output_particle_name: str
   define the starting seed (fallback_*) and the current seed output_particle_name.

These arguments are never used and they always correspond to default values.

What reco modules do now is write the starting seed to the default value of input_(pos|time)_name (server-side) which is cfg.INPUT_(POS|TIME)_NAME and read the current seed from the default value of output_particle_name (client-side) which is cfg.OUTPUT_PARTICLE_NAME.

In other words, there is no benefit in defining any of this in start_scan. The reco module could provide a name for the starting seed (optionally, more than one) while output_particle_name is purely conventional.

[tianluyuan]

@mlincett are you able to get to this still?

[mlincett]

@mlincett are you able to get to this still?

I'd like to, but I have some other things on my backlog still.