reconstruct at location

We should also add an option to allow the user to specify locations as 3D coordinates. that would allow them to specify location that weren't already in the model, which would make the syntax more general. e.g. a future version could work as follows:

if recon_loc_inds is None, reconstruct at all model locations
if recon_loc_inds is an array, reconstruct at those inds in the Model object
if recon_loc_inds is a matrix with 3 columns, reconstruct at those MNI locations (even if they're not in the model; may require blurring out the model to those locations)

The issue names no files or tests; start by locating the reconstruction entry point that handles recon_loc_inds and the Model location representation. Check the existing behavior for None and index arrays, then define coverage for a three-column MNI-coordinate matrix, including locations absent from the model. Done means all three input forms are supported and tested.

Written by the indexing model from the issue text.