Slice viz overhaul

Did multiple enhancements of the the slice_viz function. Short overview:

• Changed name from slice_viz to slices to avoid calling qim3d.viz.slice_viz (i.e. 'viz' occuring twice)
• Fixed integer rounding error with np.linspace
• Removed possibility to pass strings as input that would then be loaded with qim3d.io.load
• Added support for viewing slices in an arbitrary dimension
• Changed meaning of kwarg 'axis' from being used to if axis not True: ax.axis('off')to determining which dimension that will be sliced through. This means thatax.axis('off')` will always be called
• Added interpolation kwarg
• Added several new unit tests