ENH Brain.save_image_sequence():  add option for montage specification
ENH:  add Brain.save_movie()
EXA:  add Brain.save_movie() example
TEST Brain.save_movie()
DOC make_montage():  More descriptive error if make_montage() is called with blank image
TEST
API
DOC/API colorbar argument:  allow int
ENH:  interpolate between time points

To achieve specified framerate and time_dilation in Brain.save_movie()
API Brain.save_movie():

- parameter order
- rm `orientation`
- `interpolation` default -> ‘quadratic’
- `montage` DOC
API Brain.save_movie():  remove `col` and `row` parameters
ENH montage:  allow str
API Brain.save_movie():  remove `montage` option
DOC Brain.save_movie():  note on FFmpeg
DOC: interpolation

- list different methods
- be explicit about when it's applied
simplify FFmpeg wrapper API
API movies/FFmpeg:  default framerate=24
FIX has_ffmpeg()
STY:  flake8
STY example