Better naming scheme for workdir iteration directories

[TheChymera]

One thing that got lost in the recent preprocessing update is that the workdir is now subdivided in what appears to be indices of the BIDS dataframe, rather than more easily explorable names such as <subjectID>-<sessionID>.

It'll be rather difficult for users to efficiently debug workflows in the future if we can't locate the directories based on details which we otherwise use to identify the data:

chymera@clusterhost ~/ni_data/ofM.dr $ ls bids/preprocessing/generic_work/*
bids/preprocessing/generic_work/d3.js        bids/preprocessing/generic_work/graph.dot      bids/preprocessing/generic_work/graph.json
bids/preprocessing/generic_work/graph1.json  bids/preprocessing/generic_work/graph.dot.png  bids/preprocessing/generic_work/index.html

bids/preprocessing/generic_work/_ind_type_402:
bids_filename  bids_stim_filename  dummy_scans  events_file  f_biascorrect  f_register  get_f_scan  get_s_scan  s_biascorrect  s_bids_filename  slicetimer  s_register  temporal_mean

bids/preprocessing/generic_work/_ind_type_421:
bids_filename  bids_stim_filename  dummy_scans  events_file  f_biascorrect  f_register  get_f_scan  get_s_scan  s_biascorrect  s_bids_filename  slicetimer  s_register  temporal_mean

bids/preprocessing/generic_work/_ind_type_576:
bids_filename  bids_stim_filename  dummy_scans  events_file  f_biascorrect  f_register  get_f_scan  get_s_scan  s_biascorrect  s_bids_filename  slicetimer  s_register  temporal_mean

bids/preprocessing/generic_work/_ind_type_622:
bids_filename  bids_stim_filename  dummy_scans  events_file  f_biascorrect  f_register  get_f_scan  get_s_scan  s_biascorrect  s_bids_filename  slicetimer  s_register  temporal_mean

bids/preprocessing/generic_work/_ind_type_765:
bids_filename  bids_stim_filename  dummy_scans  events_file  f_biascorrect  f_register  get_f_scan  get_s_scan  s_biascorrect  s_bids_filename  slicetimer  s_register  temporal_mean

@damaggu could you address this (otherwise the workflow seems to be working nicely)