Explicit reference of parameter vector

[Lestropie]

Currently, "OrientationRepresentation": "param" is inadequately defined.

Either:

1. "param" must be replaced with something that can be cross-referenced in the specification that describes how to interpret the different volumes;
2. If "param" is specified, there must be a different metadata field that then provides some form of information about what parameters are contained within that file.

I had thought that 2 may be more generic and therefore more amenable to extension, but in retrospect I'm not convinced that it actually does so, and so 1 would be simpler.

So the list of data representations would need to be expanded to include symmetric rank-2 tensor. We would need to contemplate whether the metadata value would simply be "tensor", relying on the inference of antipodal symmetry (and perhaps if "AntipodalSymmetry" were true, the image would be expected to have 9 volumes rather than 6), or whether the symmetry of the tensor should be indicated in the OrientationRepresentation string.

(I'm keeping in mind that the set of orientation representations may eventually be utilised outside of DWI models...)

[Lestropie]

Arbitrary "parameter" vectors was removed in the course of producing #92.