Add read and write support of Diffusion Files

[ju-ec]

DMR and DWI file:
https://support.brainvoyager.com/brainvoyager/automation-development/84-file-formats/341-users-guide-2-3-the-format-of-dmr-files
https://support.brainvoyager.com/brainvoyager/automation-development/84-file-formats/347-users-guide-2-3-the-format-of-dwi-files

GRB (gradient file):
https://support.brainvoyager.com/brainvoyager/diffusion-weighted-imaging/61-dmr-creation/308-grb-creation-from-a-diffusion-direction-table

VDW file:
https://support.brainvoyager.com/brainvoyager/automation-development/84-file-formats/349-users-guide-2-3-the-format-of-vdw-files

DDT file (Diffusion Tensor):
https://support.brainvoyager.com/brainvoyager/diffusion-weighted-imaging/64-diffusion-tensor-ddt-files/312-ddt-file-format

[nausikaa8]

FBR file (fiber file):
https://helpdesk.brainvoyager.com/603922-The-Format-of-FBR-Files

[nausikaa8]

I'm working on the FBR format now.
For diffusion files, see also issue #25.