Replace pickled splines with other data serialization method

[scottstanie]

Noting this issue for whichever improved method we use to save the carballo cost lookup table: The Scipy RegularGridInterpolator objects have changed in recent versions, leading to this unwrapping error:

  File "/Users/staniewi/repos/dolphin/src/dolphin/unwrap/_unwrap.py", line 425, in unwrap
    unw_path, conncomp_path = unwrap_whirlwind(
                              ^^^^^^^^^^^^^^^^^
  File "/Users/staniewi/repos/dolphin/src/dolphin/unwrap/_whirlwind.py", line 99, in unwrap_whirlwind
    unw = ww.unwrap(igram, corr, nlooks, mask=mask)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/staniewi/miniconda3/envs/mapping-312/lib/python3.12/site-packages/whirlwind/_unwrap.py", line 28, in unwrap
    cost = compute_carballo_costs(igram, corr, nlooks, mask)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/staniewi/miniconda3/envs/mapping-312/lib/python3.12/site-packages/whirlwind/_cost.py", line 74, in compute_carballo_costs
    cost_up = compute_cost(-phase_dx_smooth, corr_dx)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/staniewi/miniconda3/envs/mapping-312/lib/python3.12/site-packages/whirlwind/_cost.py", line 65, in compute_cost
    p1_batch = spline_pdf1((phase_batch, corr_batch, nlooks))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/staniewi/miniconda3/envs/mapping-312/lib/python3.12/site-packages/scipy/interpolate/_rgi.py", line 391, in __call__
    _spline = self._spline
              ^^^^^^^^^^^^
AttributeError: 'RegularGridInterpolator' object has no attribute '_spline'