No 2D or 3D consistency required in the sourcePos-detectorPos pair specifications

[HanBnrd]

Also related to this, I think that nothing prevents from having for example only sourcePos3D and detectorPos2D (i.e. one is 2D and the other 3D) based on current notations, could that be an issue?
That would make a new symbol requiring presence if another field is present even more necessary.

|         `sourcePos2D`                 | * Source 2-D positions in `LengthUnit`       | `[[<f>,...]]`*¹|
|         `sourcePos3D`                 | * Source 3-D positions in `LengthUnit`       | `[[<f>,...]]`*¹|
|         `detectorPos2D`               | * Detector 2-D positions in `LengthUnit`     | `[[<f>,...]]`*²|
|         `detectorPos3D`               | * Detector 3-D positions in `LengthUnit`     | `[[<f>,...]]`*²|

 `*ⁿ` in the last column indicates that at least one of the subfields in the subgroup identified by `n` is required

Originally posted by @HanBnrd in #163