Storage formats for exported intermediate data

[Lestropie]

1. If tractograms are to be provided with just streamline endpoints, the .tck format is highly sub-optimal: it stores a NaN triplet as a delimiter between streamlines, so with exactly two vertices per streamline it increases storage by 50%.

2. Quantitative values per streamline are currently stored in ASCII form. Using binary floating-point (with a guaranteed endianness) would require less space. For fixel data we actually just use .mif images that are 1D, and that permits flexible endianness and memory-mapping; we could do that here, or something different. But either way, it may require some development at the MRtrix3 end to allow using those data as input to tck2connectome.

[sina-mansour]

This is now implemented so that the endpoints from tractograms and all quantitative values per streamlines are stored as NumPy binaries (NPY).