NiftiMapsMasker

The equivalent of NiftiLabelsMasker for maps has been added. Nothing fancy, it is mostly a copy-paste of NiftiLabelsMasker. 

The output value of `img_to_signals_maps` has been slightly changed: it now returns a list of label in addition of the signals themselves. This was for consistency with `img_to_signals_labels`. Some tests on these functions have been added as well.