[ENH] Add a FramewiseDisplacement interface

[oesteban]

Calculates the FD following the implementation of fsl_motion_outliers,
that theoretically follows that of Power et al. 2012.

If save_plot is True, then it will save an FD plot using
seaborn (but this is optional). An example of this FD plot is here:

Includes a test which assesses that this implementation is equivalent
to the one in fsl_motion_outliers

[coveralls]

Coverage decreased (-0.03%) to 72.235% when pulling 54a6acb on oesteban:enh/FDInterface into 99d191a on nipy:master.

[coveralls]

Coverage decreased (-0.03%) to 72.235% when pulling 54a6acb on oesteban:enh/FDInterface into 99d191a on nipy:master.

[coveralls]

Coverage decreased (-0.03%) to 72.235% when pulling 54a6acb on oesteban:enh/FDInterface into 99d191a on nipy:master.

[chrisgorgo]

It would be great to add option to plot and/or return the values in m/s (speed) instead of mm.

For plotting this could be just another axis (right) labelling.

[oesteban]

If you set normalize=True then the plot is done in mm/s and the legends read correctly. But I can do what you propose.

[chrisgorgo]

My bad I missed that.

[coveralls]

Coverage decreased (-0.02%) to 72.246% when pulling 98b522c on oesteban:enh/FDInterface into 99d191a on nipy:master.

[coveralls]

Coverage decreased (-0.02%) to 72.245% when pulling ee3676c on oesteban:enh/FDInterface into 99d191a on nipy:master.

[coveralls]

Coverage decreased (-0.02%) to 72.245% when pulling ee3676c on oesteban:enh/FDInterface into 99d191a on nipy:master.

[coveralls]

Coverage decreased (-0.02%) to 72.245% when pulling ee3676c on oesteban:enh/FDInterface into 99d191a on nipy:master.

[coveralls]

Coverage decreased (-0.02%) to 72.245% when pulling 8708bf4 on oesteban:enh/FDInterface into 0f9a620 on nipy:master.

[oesteban]

@satra, @chrisfilo I think this one is ready to go