Let users pass a table of coordinates instead of an atlas file

[jdkent]

Is your feature request related to a problem? Please describe.
If I have motivated coordinates to investigate, there is additional friction to create a nifti image with spheres around the coordinates before passing it into nibs.

Describe the solution you'd like
Be able to read in a table of coordinates like what nistats' get_clusters_table function outputs.

Describe alternatives you've considered
Continue making the nifti atlases beforehand

Additional context
N/A