Generate multiple surfaces

[oesteban]

Convert this node in a mapnode that iterates over several depths. cc @satra for the actual values we need to iterate over

https://github.com/poldracklab/smriprep/blob/a40cf9060960fcda2c3ecd21ab9add1f3bb8079d/smriprep/workflows/surfaces.py#L437

[satra]

given current resolutions and assuming cortex is empirically around 3mm thick, i would suggest we do 0.1, 0.25, 0.5, 0.75, 0.9 by default, with a flag to allow overriding this list. the list would depend on the use cases.