Add Feature: Signal to Noise Maps

[hebbianloop]

Additional quantitative summary, use DenoiseImage or other software (nipype?) to generate SNR + CNR maps.

Feature should write to volume and surface files. Surfaces should be included for web-report for individual subjects + average. Parametric maps should be 12dof registered to average first, then used to generate dataset summaries.

Additional feature would include ability to sample SNR/CNR at specific parcels using given atlas.