ENH: Add a script to plot the signal estimated by the GP

Add a script to plot the signal estimated by the GP as well as the error
data generated by the error analysis script.

Add the necessary helper functions to the signal visualization module to
plot the estimated signal as a surface.

Modify the signal visualization error plotting method to optionally
accept the color the figure size parameters.

Add methods to the the signal simulation module in order to serialize
the dMRI data.

Refactor the the the signal simulation module to:
- Allow the dMRI signal generation method to generate evals randomly if
  not provided.
- Allow reusing the polar random angle generation utility.
- Allow the single tensor method to accept a random generator for the
  sake of reproducibility.
- Set the `zip` function `strict` parameter to `True` as we want all
  iterables to have the same length.

Modify the error analysis script to:
- Reuse the `EddyMotionGPR` instance: factor it out from the CV
  function, as the instance does not change across folds and repeats.
- Save the simulated signal and gtab.
- Predict and save the signal of the GP estimation.
- Save the simulated SNR to the CV scores data file. Since `None`
  indicates no noise, modify the `pandas` serialization method arguments
  so that `None` is not considered as a missing value.

Take advantage of the commit to rename the `evals1` argument to `evals`
in the error analysis script.

src/eddymotion/testing/simulations.py

@@ -31,7 +31,11 @@
 from dipy.core.sphere import HemiSphere, Sphere, disperse_charges
 from dipy.sims.voxel import all_tensor_evecs, single_tensor
 
+<<<<<<< HEAD
 # Bounds defined following Canales-Rodriguez, NIMG 184 2019, https://doi.org/10.1016/j.neuroimage.2018.08.071
+=======
+# Set according to Canales-Rodriguez, NIMG 184 2019, https://doi.org/10.1016/j.neuroimage.2018.08.071
+>>>>>>> 73963e0 (ENH: Add a script to plot the signal estimated by the GP)
 BOUNDS_LAMBDA1: tuple[float, float] = (1.4e-3, 1.8e-3)
 BOUNDS_LAMBDA23: tuple[float, float] = (0.1e-3, 0.5e-3)