NEWFEATURE: get_cut_coords(...) function to heuristically compute good cross-sectional cut_coords

In the plot_map(...) API, if `cut_coords='auto'` is specified and `slicer`is one of `'x'`, `'y'`, `'z'`, then a series of 'cross-sectional' `cut_coords` is generated and used (similar to FSL activation maps).

* localizer_glm_ar.py demo script now uses this feature