[ENH] Update Warp datatype and logic

[fraimondo]

1. Modify the Warp datatype to be a "list" of mappings from / to spaces.
2. Add a "warper" key to the Warp datatype indicating which warper was used: fsl or ants for the moment.
3. Modify the SpaceWarper to allow to "auto" select the warper based on the dataset. This should require both FSL and ANTs as external dependencies. This only works if reference is "T1w": "auto" is not defined for junifer internal transforms (i.e. between template spaces)
4. Modify coordinates/masks/parcellations so they check for the right transform and use the "warper" parameter accordingly.