Added new step roi_tensor

The new step roi_tensor calculates tensor measures (e.g., FA, defined in the
TENSOR_MEASURES field of dwi_analysis_settings.py) in all ROIs of a given
cortical parcellation paradigm. It does the calculation for all WM depths
specified in WM_DEPTHS of dwi_analysis_settings.py, as well as in the GM.
This step requires the completion of the tracula_prep and parcellate steps.

	new file:   diffusion_tensor.py
	modified:   dwi_analysis.py
	modified:   dwi_analysis_settings.py
	modified:   tractography.py