Manual box gating

[paulmueller]

The current Gate class only knows box gates given by input files. It might make sense to introduce the ability to set custom gating options.

• This requires changing the pipeline identifiers for Gating
• Requires a list/dict of (at least) common feature names and their abbreviation in the pipeline identifiers
• Does not require bumping the PPID generation (pipeline hash changes anyway)
• Might want to remove the online_gates keyword argument from the PPID, since box gates are overriden anyway if they are specified in PPID.