Add diagnostic output for `model.cond`

[grst]

Description of feature

When creating a complex contrast with model.cond, there should be a way to print out the actual contrast generated as a santity check for the user.

Follow-up of scverse/multi-condition-comparisions#15.

[grst]

One way could be to implement a method display_contrast(contrast: np.ndarray | pd.Series) that visualizes the contrast.

In its simplest form, it could show the non-zero coefficients and display them as a table:

contrasts.T.loc[lambda x: x[0] != 0]