Fix and extend flow derivatives and losses

[aschuh-hf]

• Add flow_derivatives() with default spacing for finite differences calculation assuming a normalized flow field.
• Add curl(), divergence(), and divergence_free_flow() functions to core library.
• Modify finite_differences() to pad input using replicate instead of padding output with zeros.
• Add mode='forward_central_backward' to avoid padding using replicate or constant values.
• Add mode='bspline' to spatial_derivatives().
• Generalize cubic_bspline_jacobian_*() to jacobian_*() functions for all mode values.
• Fix bending energy, curvature, and divergence loss functions.

Fixes #113, #114, #115, and #116.