ENH: Add mask_vals function

[muschellij2]

vals = mask_vals(img, mask)
mask_vals(img, mask) <- vals

which is pretty much:

mask_vals = function(img, mask) {
img = check_nifti(img)
mask = check_nifti(mask)
check_mask_fail(mask)
vals = img_data(img)[ mask %in% 1]  # == or %in% ?
return(vals)
}