`scil_dti_metrics` should check for shell validity

[AlexVCaron]

Current behavior

• scil_dti_metrics eats its input dwi, bvals and bvecs as is and tries to computes.

Expected behavior

1. scil_dti_metrics eats its input dwi, bvals and bvecs
2. It checks bvals, up to tolerance, bins them up in ranges, assert there is more than a bval == 0 shell
3. Either computes, or send an error to the CLI we can catch

Alternative implementation

Instead of going down to scil_dti_metrics, we can catch early in scil_dwi_extract_shell. Options :

• Parameter to hard request shells and if some misses, output an error code
• Output shells extracted, for comparison against the shells wanted

[AlexVCaron]

Addresses nf-neuro/modules#147