Added proper handling of nifti RGB formats

[baishi]

Two formats of NIfTI are properly supported

1. The format of (x, y, z, t, (uint8, uint8, uint8)) is supported and mmap works out of the box
2. The format of (x, y, z, t, [('R', 'u1'), ('G', 'u1'), ('B', 'u1')]) structured rgb format is also supported. However, due to limitation of nibabel, the full dataset need to be loaded into memory.
   A warning is printed for this case

[xgui3783]

@baishi thanks for the PR.

Is there a sample nifti where we can write test to prevent regression in the future?

[baishi]

@baishi thanks for the PR.

Is there a sample nifti where we can write test to prevent regression in the future?

sample_tuple.nii.gz

sample_structure.nii.gz

I have attached two sample image to test against.
sample_tuple uses 3 tuple of 8 bits (datatype 2) on 5th dimension (dim[5] a.k.a x, y, z, t, rgb) to store the value
and
sample_structure uses 1 value of 24-bit (datatype 128) on 5th dimension to store the structured rgb value.