Replace `mri_convert --conform` with nibabel version

[kaczmarj]

Nibabel has a function almost equivalent to mri_convert --conform: https://github.com/nipy/nibabel/pull/853

The main difference is that the nibabel version does not implement histogram normalization. The histogram normalization code is translated to python in https://github.com/nipy/nibabel/pull/853#issuecomment-616688636 and can be implemented with numpy.histogram.