Renamed "target" into "resampling_target"
Simplified examples.
Added resampling to NiftiLabelsMasker

resampling_target parameter has been added to NiftiLabelsMasker. Only one
target has been implemented: labels, because resampling labels is considered
dangerous right now.

Corrected a bug in NiftiMapsMasker: the interpolation method used for "maps"
and "mask" targets were swapped.