[BUGFIX] Enable Cumulative Curl model to work with a cubature grid

[misi9170]

As raised in #970 and originally in #709 , the Cumulative Curl model does not currently work with a cubature grid. It took me quite a while to track the issue down, but finally, the solution was simply to add dimensions to turb_avg_vels for use in the initial computation of thrust_coefficient and axial_induction.

@Bartdoekemeijer kindly provided a test script:

from floris import FlorisModel

fmodel = FlorisModel("inputs/cc.yaml")
fmodel.set(solver_settings={"type": "turbine_cubature_grid", "turbine_grid_points": 3})
fmodel.run()
print(fmodel.get_turbine_powers())

On the develop branch, this fails, producing the error reported in #970. After this bugfix, the code runs as expected, producing

[[1746971.6830192   471716.37117722  467636.89587481]]

Moreover, commenting out the line fmodel.set(solver_settings={"type": "turbine_cubature_grid", "turbine_grid_points": 3}) (which is to say, using the "standard" square grid) produces

[[1753954.45917917  381082.81148362  446768.73505829]]

on both the develop branch and this bugfix branch (that is, no change to results when not using a cubature grid). The difference in powers between the cubature grid and square grid is expected.

All tests pass and this PR is ready for review and merger.