TEST scale_mri:  use smaller source space and python setup_source_space()
DOC add_source_space_distances() error message
TEST scale_source_space() with distances (BK)
FIX scale_source_space():  add distances with 3 scaling params
OPT TEST:  test_scale_mri()
OPT TEST test_scale_mri():  delete all but one label file
DOC:  whats-new
Merge pull request #1478 from christianbrodbeck/fix-scale-ss

Fix scale_source_space
more informative warning
update manifest
ADD digital mask option
ADD test
FIX masking to block matching ones
STY and PEP8
Make default zero
PEP8 and ENH error message
UPDATE whats_new