Handling multi-echo edge cases

[oesteban]

This issue is a split of #1803 to address this #1803 (comment). We identified two issues:

• Using the --echo-idx <idx> flag to process a particular echo. This case would be correctly handled at the moment, although the logged messages would be inaccurate. This situation should be correctly logged. That said, I'd propose to deprecate the --echo-idx <idx> argument in favor of --bids-filters. Comments?
• Multi-echo, but only two echos present. The collect_data utility should not group echoes in this case.

I'm assigning this to target 20.2, but we may push it away to 20.3 if needed.

[oesteban]

Okay, @emdupre had already considered the dual-echo case (https://github.com/nipreps/niworkflows/blob/master/niworkflows/utils/bids.py#L251-L252).

Closing...