Add getter for number of total voxels to NiftiImageData

[johannesmayer]

This function is useful to loop over the elements.

[rijobro]

Merged. Just for interest, I was using get_raw_nifti_sptr()->nvox before.

[KrisThielemans]

can I ask how you're using this?

If we need it, we should have a virtual function higher up (which could just get it from the geo_info)

[rijobro]

Apologies for merging, do you want me to revert?

Although we should be using geom_info, I don't fully trust it quite yet (e.g., varying slice distances for MR images). Should we make this an issue to solve in the future?

[johannesmayer]

I decided to load the segmentation input of my simulation into a 'NiftiImageData3D' (no modality attached, just geometry and image information).

At some points I want to loop over all the pixels in the image and assign them to a specific tissue type.
I didn't see the functionality which Richard just suggested, so my code became very messy.

Don't think personally I will use it very often on top of that.