`Object3d.flatten()` flattens in the opposite order from numpy

[argerlt]

Messing around with #588 and #593, I've realized Object3d.flatten() flattens in column-major (Fortran-style) order, as opposed to the numpy default of row-major (C-style). more details here:

The initial fix is easy, just replace the existing function with the following:

    def flatten(self) -> Object3d:
        """Return a new object with the same data in a single column."""
        obj = self.__class__(self.data.reshape(self.size, self.dim))
        return obj

However, this breaks 120-ish tests, and likely some downstream applications as well.

Is this worth fixing? I'm down to take care of it, I just want to check in with others in case there is a good reason to leave this as is.

I'll also note, this ONLY is an issue when you have paired data in ORIX objects and non-orix arrays, as all ORIX objects inherit from Object3d, and are thus self-consistent

Example of the flattening issue in action:

from orix.vector import Vector3d
import matplotlib.pyplot as plt
import numpy as np

x, y = np.meshgrid(np.linspace(0, 1, 100), np.linspace(0, 1, 100))
v = Vector3d(np.stack([x, y, y*0], axis=2))
fig, ax = plt.subplots(1, 2)

# When flattened by ORIX, colum-major
ax[0].imshow(v.flatten().reshape(100, 100).x)
ax[0].set_title("Column-major flattening via ORIX")

# When flattened by numpy, row-major, which flips the axes.
ax[1].imshow(v.x.flatten().reshape(100, 100))
ax[1].set_title("Row-major flattening via numpy")
plt.tight_layout()

[hakonanes]

Thank you for pointing this out.

There's no reason for orix to differ from NumPy. As you say, changing it directly breaks tests. How about something like this as a plan:

1. Add an order: Literal["F", "C"] | None = None parameter, just like NumPy. F = Fortran-order = column-major, and C = C-order = row-major is the other option. If no order is given, "F" is used, maintaining current behavior.
2. Try to use C-order internally as many places as possible without changing external behavior (breaking the API)
3. If no order is given, warn that row-major order will become default in the future. If an order is given, that is of course used, and no warning is given. Users giving the order does not need to be warned that the default order will change in the future.
4. After some time (two minor releases?), change to order: Literal["C", "F"] = "C".