make a slice_index to slice_value convertor

[shimwell]

need to be able to convert from slice index to the axis value (slice value)

lower_left and upper_right should give the axis range then we can relate index and a view direction and dimension into a slice value

index_val = {'x': 0,  'y':1,'z': }
vd_to_width = upper_right[index_val] - lower_left[index_val]
value = lower_left[index_val] + vd_to_width / mesh.dimension[index_val]+1  #check this part