Fix failing test_time_variable

[mcflugen]

This pull request fixes a failing test in test_strat.py. I believe it is caused by how numpy2 interacts with xarray DataArrays.

The error is caused by the following,

np.linspace(0, float(golfcube.t[-1]), num=5)

which raised the following error,

ValueError: dimensions () must have the same length as the number of data dimensions, ndim=1

I've fixed this by converting golfcube.t[-1] to a float (rather than a DataArray).

I could, instead, have done the following

np.linspace(0, golfcube.t[-1].data, num=5)

Both seem to work and are backward compatible.

[mcflugen]

@amoodie This should fix the failing test we talked about this afternoon.