FIX: Add required parameters for dti and fODF shells

[gagnonanthony]

DTI and fODF shells were previously inferred from a threshold that could be specified by --dti_max_shell_value and --fodf_min_fodf_shell_value. However, I find this unstable and not customizable by the user. Therefore, the user will not have to specify which shell to use for both modeling methods using --dti_shells and --fodf_shells.

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